Optical Properties of ZnO Nanostructures

reviews

A. B. Djurisˇic´ and Y. H. Leung

Zinc oxide DOI: 10.1002/smll.200600134

Optical Properties of ZnO Nanostructures Aleksandra B. Djurisˇic´* and Yu Hang Leung

From the Contents 1. Introduction............. 945 2. Spontaneous Emission ................................ 947 3. Stimulated Emission 951 4. Nonlinear Optical Properties................ 955 5. Optical Properties of Doped ZnO............... 956 6. Conclusions and Outlook.................... 957

Keywords: Many morphological variations of nanostructured ZnO lead to some interesting optical properties.

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www.small-journal.com

2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

· nanostructures · photoluminescence · spontaneous emission · stimulated emission · zinc oxide

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Optical Properties of ZnO Nanostructures

W

e present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry–Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as secondharmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.

1. Introduction Zinc oxide is a material with great potential for a variety of practical applications, such as piezoelectric transducers, optical waveguides, surface acoustic wave devices, varistors, phosphors, transparent conductive oxides, chemical and gas sensors, spin functional devices, and UV-light emitters.[1, 2] Its wide bandgap ( 3.37 eV at room temperature[1]) makes ZnO a promising material for photonic applications in the UV or blue spectral range, while the high exciton-binding energy (60 meV)[1] allows efficient excitonic emission even at room temperature. In addition, ZnO doped with transition metals shows great promise for spintronic applications.[3] It has also been suggested that ZnO exhibits sensitivity to various gas species, namely ethanol (C2H5OH), acetylene (C2H2), and carbon monoxide (CO), which makes it suitable for sensing applications. Moreover, its piezoelectric property (originating from its non-centrosymmetric structure) makes it suitable for electromechanical sensor or actuator applications. Also, ZnO is biocompatible which makes it suitable for biomedical applications. Last but not least, ZnO is a chemically stable and environmentally friendly material. Consequently, there is considerable interest in studying ZnO in the form of powders, single crystals, thin films, or nanostructures. A variety of ZnO nanostructure morphologies, such as nanowires,[4–9] nanorods,[10–14] tetrapods,[14–18] and nanoribbons/belts,[6, 18–21] have been reported. ZnO nanostructures have been fabricated by various methods, such as thermal evaporation,[4–6, 9, 14–21] metal–organic vapor phase epitaxy (MOVPE),[12] laser ablation,[13] hydrothermal synthesis,[7, 10, 11] and template-based synthesis.[8] Recently, novel morphologies such as hierarchical nanostructures,[22] bridge-/nail-like nanostructures,[23] tubular nanostructures,[24] nanosheets,[25] nanopropeller arrays,[26, 27] nanohelixes,[26, 28] and nanoACHTUNGRErings[26, 28] have, amongst others, been demonstrated. Some of the possible ZnO nanostructure morphologies are shown small 2006, 2, No. 8-9, 944 – 961

in Figures 1–3. Several recent review articles have summarized progress in the growth and applications of ZnO nanostructures.[29–31] The growth and properties of ZnO nanostructures have been extensively studied,[32–75] but there are still a number of unanswered questions concerning the relationship between fabrication conditions and optical properties. The fabrication methods for ZnO nanostructures can be divided into two groups: spontaneous growth and templatebased synthesis (for example, using an alumina template). Fabrication without a template can occur either by using metal catalysts or may be self-catalyzed. The use of metal catalysts, such as Au, can be an advantage for achieving aligned and selective area growth.[4] Aligned nanorods can also be obtained by a hydrothermal method without any metal catalyst.[7, 46] The degree of alignment and the achieved aspect ratio was dependent on the seed layer used and the fabrication conditions.[7, 46] An improvement in alignment of the rods perpendicular to the substrate was obtained when zinc acetate was used to prepare the nanocrystalline seed layer instead of ZnO nanoparticles.[7] The growth of ZnO by vapor deposition is typically affected by temperatures of the source and the substrate, the distance between the source and the substrate, the heating rate, the gas flow rate, tube diameter, and the starting precursor(s).[75] The influence of these factors on ZnO morphology was

[*] Dr. A. B. Djurisˇic´ Department of Physics, The University of Hong Kong Pokfulam Road (Hong Kong) Fax: (+ 852) 2559-9152 E-mail: [email protected] Y. H. Leung Department of Chemistry, The University of Hong Kong Pokfulam Road (Hong Kong)


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Figure 2. a–d) Representative scanning electron microscopy images of ZnO nanopropeller arrays. Reprinted with permission from Ref. [27].

Figure 1. a–f) Representative scanning electron microscopy images of various ZnO nanostructure morphologies.

studied in detail recently,[75] but the effects of these factors on the optical properties of fabricated nanostructures are still unknown. Different experimental conditions, such as

Aleksandra B. Djurisˇic´ obtained her PhD degree in electrical engineering from the School of Electrical Engineering at the University of Belgrade (now Serbia) in 1997. After finishing her PhD studies, she worked as a postdoctoral fellow at University of Hong Kong and as an Alexander von Humboldt postdoctoral fellow at TU Dresden. She has been assistant professor in the Dept. of Physics at the University of Hong Kong since 2003. Her research interests include the optical properties of materials, nanoACHTUNGREmaterials, wide-bandgap semiconductors, block copolymers, and optoACHTUNGREelectronic devices. Y. H. Leung obtained his B.Eng. degree from the City University of Hong Kong in 2003 and his M. Phil. degree from the University of Hong Kong in 2005. He is currently working as a research assistant at the University of Hong Kong. His research interests include the fabrication, characterization, and applications of ZnO nanostructures.

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Figure 3. A–D) Representative scanning electron microscopy images of ZnO helical nanobelts. Reprinted with permission from Ref. [28].

for example, solution concentration, temperature, and substrate pretreatment, also affect the growth of ZnO by hydrothermal methods. Due to the low growth temperature (typically under 100 8C), the crystalline quality of such samples is often lower than those fabricated by vapor deposi-


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tion. However, the optical quality of the samples can be improved by annealing under appropriate conditions.[46] In this Review, we provide a detailed overview on the optical properties of ZnO nanostructures. The Review is organized as follows: In the next section, we discuss spontaneous emission from ZnO nanostructures. Low-temperature and room-temperature photoluminescence in the UV and visible spectral regions are discussed. Next, an overview of stimulated emission in various ZnO nanostructures is provided. In chapter 4, the nonlinear optical properties of ZnO are presented. Finally, some conclusions and an outlook for the future are given.

ture PL spectrum of highly faceted ZnO rods with good crystalline quality[41] is shown in Figure 5. Since the relative intensity of the bound-exciton peaks varies from sample to sample due to variations in donor/acceptor concentrations

2. Spontaneous Emission Optical properties of a variety of forms of ZnO, including ZnO nanostructures, have been studied by photoluminescence (PL) spectroscopy.[32–122] The majority of the reported luminescence spectra of ZnO nanostructures have been measured at room temperature, although variable-temperature photoluminescence studies have been performed on some of the samples.[32–46] Room-temperature PL spectra of ZnO typically consist of a UV emission and possibly one or more visible bands due to defects and/or impurities.

2.1. UV Emission Low-temperature photoluminescence measurements of different nanostructures, such as nanowire/nanowall systems,[32] nanosheets,[33] nanowalls,[44] nanowires,[34, 43, 45] nanorods,[35, 37, 39, 46] faceted nanorods,[41] nanoparticles,[42] and nanoblades and nanoflowers,[40] have been reported. Lowtemperature (4–10 K) PL spectra of ZnO typically exhibit several peaks (labeled I0–I11), which correspond to bound excitons.[76] An example of a low-temperature PL spectrum of a ZnO sample exhibiting a number of bound-exciton peaks is shown in Figure 4. The number of observed boundexciton peaks in ZnO nanostructures is typically lower than that in ZnO single crystals. An example of a low-tempera-

Figure 4. Bound-excitonic region of the 10 K PL spectrum for the forming gas annealed ZnO substrate. Reprinted with permission from Ref. [77]. small 2006, 2, No. 8-9, 944 – 961

Figure 5. PL spectra from ZnO faceted rods at different temperatures as a function of a) wavelength, and b) energy. The inset shows an enlarged region of the PL spectrum at 7 K. Reproduced from Ref. [41].

and their capture cross sections,[77] variable-temperature PL measurements can provide useful information about the optical and structural properties of ZnO. However, the assignment of the bound-exciton peaks in ZnO is, in general, controversial for all forms of the samples, namely, ZnO single crystals, epitaxial films, and nanostructures. For example, it was proposed that the emission lines I5 to I11 in the lower part of the energy spectrum can be attributed to excitons bound to neutral acceptors.[78] However, other reports in the literature attributed some of these lines to donor bound excitons.[76, 79] The chemical identity of the donors and acceptors responsible for different bound-exciton lines still remains unclear (for a complete list of the bound-exciton peaks generally observed in ZnO, and a summary of the possible identification of the donors and acceptors, see Refs. [1, 76]). One of the commonly observed bound-exciton lines in ZnO nanostructures is the I4 line at 3.3628 eV.[41, 82] This emission is typically attributed to the donor bound exciton, and the donor has been identified as hydrogen.[76, 80, 81] Theoretical calculations predict hydrogen to be a shallow donor in ZnO[123] and it is reasonable to expect that an unintentional incorporation of hydrogen could frequently happen in ZnO nanostructure synthesis. While in general there is a consensus in assigning the I4 line to hydrogen donors,[76, 80, 81] the chemical identity of donors responsible for other donor bound-exciton lines remains unclear. For the acceptor bound excitons, the most commonly reported peak is located at 3.3564 eV.[77] This peak is commonly attributed to excitons bound to Na or Li acceptors.[1] Alkali metals are predicted to produce shallow acceptors on the cation site, but the experimental results demonstrate that doping with group 1 ions produces complex results.[124]



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However, other acceptor levels have also been proposed, such as an acceptor complex involving a N impurity on an O site.[81] However, some authors attribute this line to a donor bound exciton instead.[76, 79] Bound-exciton lines I6, I8, and I9 have been assigned to excitons bound to Al, Ga, and In donors, respectively.[76] On the other hand, Thonke et al.[82] proposed that the weak 3.357 eV line corresponds to the acceptor bound exciton, while the I8 line at 3.3597 eV was found to be a donor bound-exciton line. In addition to commonly observed acceptor bound-exciton lines, emission at 3.332 eV (labeled as Ia) was recently observed in low-temperature PL spectra of ZnO epilayers grown on CaF2ACHTUNGRE(111).[109] Since this peak occurs in the spectral region where two-electron satellites (TES) of donor bound-exciton peaks are expected to occur,[82] careful examination of the peak position in respect to known boundexciton positions and expected TES peaks (see Ref. [76] for the positions of TES lines for different bound-exciton peaks) is necessary. In addition, the occurrence of peak near 3.333 eV may indicate excitons bound to structural defects.[76] Therefore, further work is needed for conclusive identification of the origin of different bound-exciton lines in ZnO. The assignment of several bound-exciton lines, especially I9, is still controversial and conclusive chemical identification of the majority of donors and acceptors has not been accomplished. At low temperatures, in addition to bound-exciton peaks, two-electron satellite transitions can be observed in the spectral region 3.32–3.34 eV.[77] These transitions correspond to a radiative recombination of donor bound excitons, which leaves the donor in an excited state. Thus, they are located at an energy lower by an amount equal to the difference between the first excited and ground states of the donor, so that their position in respect to the donor boundexciton peaks can be used to estimate donor-binding energies.[82, 83] Finally, low-temperature PL spectra can also contain donor–acceptor pair transitions and longitudinal optical (LO) phonon replicas.[77] First-, second-, and third-order LO phonon replicas can typically be observed.[44] The LO phonon energy can be determined from the separation between the exciton peaks and their LO phonon replicas, and for ZnO it is 71–73 meV.[45, 84] Since donor–acceptor pair transitions and some of the LO phonon replicas occur in the same spectral region (3.218–3.223 eV),[77] care needs to be taken in assigning the peaks observed in this region. In addition, two-phonon replicas due to two transverse optical phonons (separation of 108 meV) were also reported in ZnO thin films prepared by spray pyrolysis.[113] With regard to the temperature dependence of the observed peaks, a red shift of the free-exciton emission with increasing temperature occurs. The intensity of the boundexciton peaks and the LO phonon replicas decreases with increasing temperature, and only free-exciton emission can be observed at room temperature. In ZnO epilayers, freeexciton emission was found to dominate the spectra above 70 K.[109] Similar behavior, with the disappearance of bound-exciton peaks above 150 K, was also observed in ZnO single-crystal samples.[110] The bound-exciton line for

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ZnO nanoparticles embedded in alkali halide crystals also disappeared at 125 K.[115] The exact temperature at which the bound-exciton line will disappear depends on the identity of the donors or acceptors, since different donors/acceptors will be thermally ionized at different temperatures. It should be noted that in the case of donor–acceptor pair transition, disappearance of this peak with increasing temperature can be accompanied by the appearance of acceptor bound-exciton peaks if the acceptors are thermally ionized at higher temperature than the donors.[117] However, what all ZnO samples (single crystals, films, and nanostructures) have in common is the disappearance of bound-exciton peaks at temperatures in the range 50– 150 K, while at room temperature only free-exciton emission is observed. The presence of free-exciton emission at low temperatures, as well as a distinction between A and B exciton peaks, is usually considered to indicate high quality in ZnO samples.[111] It should be noted that this criterion for sample quality is less arbitrary than the ratio between UV and defect emission, which is sometimes used to estimate sample quality,[116, 121] and which is dependent on excitation area and power.[74] Distinction between A and B exciton peaks is usually not possible above 160 K,[77] and the phonon replicas typically cannot be clearly resolved above 250 K.[44] In very-high-quality samples, higher-order exciton lines can also be observed at low temperatures.[118, 122] Biexciton emission was observed at 77 K in high-quality epitaxial ZnO films.[111] The biexciton binding energy was estimated to be 15 meV.[111] Biexciton emission has also been observed in ZnO nanowires[112] and nanorods.[35, 114] The obtained energy separation between exciton and biexciton peaks in ZnO nanowires was 20 meV, in good agreement with the results obtained on other forms of ZnO.[112] The reported energy separation in the case of nanorods was 18 meV,[35] and biexciton emission persisted up to 200 K.[114] Clear observation of free-exciton and biexciton lines at low temperatures is usually considered as an indication of very good sample quality.[35] In room-temperature PL spectra, some variation of the position of the PL peak can be observed for different nanostructures. This is illustrated in Figure 6, where different UV

Figure 6. Room-temperature PL spectra of various nanostructures in the UV range: 1) Tetrapods, 2) needles, 3) nanorods, 4) shells, 5) highly faceted rods, 6) ribbons/combs.


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peak positions (387 nm for tetrapods, 381 nm for needles, 397 nm for nanorods, 377 nm for shells, 379 nm for faceted rods, and 385.5 nm for ribbons/combs) can be observed. Room-temperature band-edge emission in ZnO nanostructures was reported to occur at 373,[49] 378,[46, 53, 57] 380,[47, 64, 67] 381,[55] 383,[52, 62] 384–391,[56] 387.5,[58] 389,[50, 62] and 390 nm.[59] Individual nanostructures, such as nanobelts, exhibited UV emission in a range between 384 and 391 nm.[56] These differences in the peak positions of individual nanobelts, which are sufficiently large so that there could be no quantum confinement effects, indicate that there is likely a different explanation for the variation in the band-edge emission in ZnO nanostructures reported in different studies. Even though quantum confinement has been proposed as a cause of the blue shift of the band-edge emission with decreasing size,[69] any shift due to quantum confinement in nanocrystals with diameters of 57, 38, and 24 nm is not likely considering the fact that the Bohr radius of ZnO is 2.34 nm.[70] One possible reason for the variations in the position of the band-edge emission in various ZnO nanostructures with relatively large dimensions are different concentrations of native defects. Since the defect density on the surface is higher than in the bulk,[125] spectral shifts due to different defect concentrations are expected to occur in nanostructures with different sizes due to different surface-to-volume ratios. The fact that the decay times in time-resolved PL from ZnO nanorods are size dependent[71] is in agreement with the assumption of different defect levels/concentrations for structures with different surface-to-volume ratios. Thus, the defects could affect the position of the band-edge emission as well as the shape of the luminescence spectrum. Although there have been several reports with strong UV and weak defect emission in ZnO nanostructures,[49, 50] in some cases only defect emission is observed[48] or the UV emission is much weaker compared to the defect emission.[46] Therefore, clarifying the origins of different defect emissions is an important issue. However, it should be noted that the ratio of the intensity of UV and defect emission is dependent on the excitation density,[36, 74] as well as the excitation area.[74] Thus, the ratios of these two emissions cannot be used as an absolute determining factor of the crystalline quality of ZnO, although they are useful in comparing the quality of different samples when the measurements are performed under identical excitation conditions.

2.2. Defect Emissions Room-temperature PL spectra from ZnO can exhibit a number of different peaks in the visible spectral region, which have been attributed to the defect emission. Emission lines at 405, 420, 446, 466, 485, 510, 544, 583, and 640 nm have been reported (see Ref. [36] and references therein). Several calculations of the native defect levels in ZnO have been reported,[85–87, 124] as summarized in Figure 7. An example of defect emissions (normalized PL spectra) from different ZnO nanostructures is shown in Figure 8. Green emission is the most commonly observed defect emission in ZnO nanostructures,[4, 47–49, 52, 53, 56–58, 61, 62, 64, 67, 68] small 2006, 2, No. 8-9, 944 – 961

Figure 7. Illustration of the calculated defect energy levels in ZnO from different literature sources (data marked with the subscript “a” originate from Ref. [85], those marked with “b” stem from Ref. [87], and those marked with “c” originate from Ref. [86]). VZn, VZn , and VZn2 denote neutral, singly charged, and doubly charged zinc vacancies, respectively. Znio and Zni indicate neutral zinc interstitials, while Zni + denotes a singly charged zinc interstial. VOo and VO denote neutral oxygen vacancies, while VO + denotes a singly charged oxygen vacancy. Oi represents an oxygen interstitial. VOZni denotes a complex of an oxygen vacancy and zinc interstitial.

Figure 8. Room-temperature PL spectra of different nanostructures: 1) Tetrapods, 2) needles, 3) nanorods, 4) shells, 5) highly faceted rods, 6) ribbons/combs.

similar to other forms of ZnO. The intensity of the blue– green defect emission was found to be dependent on the nanowire diameter,[5, 64] but both increased[5] and decreased[64] defect emission intensity with decreased wire diameter were reported. Several different hypotheses have been proposed: Green emission is often attributed to singly ionized oxygen vacancies,[47–49, 64, 68] although this assignment is highly controversial. Other hypotheses include antisite oxygen,[56] which was proposed by Lin et al.[85] based on the band structure calculations. Green emission was also attributed to oxygen vacancies and zinc interstitials.[67] Cu impurities have been proposed as origin of the green emission in ZnO.[88] Blue-green defect emission was also reported in Cu doped ZnO nanowires.[65] However, although Cu was identified as a possible cause of green emission in ZnO,[88] this cannot explain the defect emission in all ZnO nanostructure samples, especially those where defect emission exhibits strong dependence on annealing temperature and atmosphere which would be more consistent with an intrinsic defect rather than Cu impurity. Other hypotheses include



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various transitions related to intrinsic defects, such as donor–acceptor transitions,[89] recombination at Vo** centers (where these centers are generated by surface trapping of photogenerated holes, followed by recombination with electron in an oxygen vacancy Vo*),[90, 91] zinc vacancy,[92, 93] and surface defects.[36] Although the singly ionized oxygen vacancy[94] is a commonly cited hypothesis, which is supported by reports of the enhancement of the green defect by annealing at temperatures above 600 8C (attributed to out-diffusion of O),[47] this assignment has been questioned recently.[36, 88] The donor–acceptor transition hypothesis used to explain the green and yellow emissions has also been challenged.[95] On the other hand, while the Zn vacancy hypothesis is supported by the study of the effect of O and Zn implantation,[93] a blue rather than green emission would be expected based purely on the theoretically predicted energy levels for Zn vacancy.[86] Therefore, the origin of the green emission is still an open and controversial question and the identification of the exact origin of this emission requires further study. While the type of defect responsible for the green emission has not yet been conclusively identified, there is convincing evidence that it is located at the surface. It was shown that coating ZnO nanostructures with a surfactant suppressed green emission.[36] Polarized luminescence experiments from aligned ZnO nanorods also indicated that green emission originated from the surface of the nanorods.[72] The surface recombination layer responsible for visible emission in ZnO nanowires was estimated to be 30 nm in thickness.[73] Also, the possible presence of Zn(OH)2 at the surface, especially for nanostructures prepared by solution methods, could affect the emission spectra from ZnO nanostructures.[51] Yellow defect emission is also commonly reported in ZnO nanostructures,[46, 62, 96] and it represents a common feature in samples prepared from aqueous solutions of zinc nitrate hydrate and hexamethylenetetramine.[46, 96] This emission is typically attributed to an oxygen interstitial,[46, 92, 96] although a Li impurity represents another possible candidate.[96] The deep levels responsible for green and yellow emissions were found to be different;[92, 96] unlike the defect responsible for the green emission, the defect responsible for the yellow emission is not located at the surface.[96] In addition to green and yellow emissions, orange-red emissions are often also observed.[53, 57, 67, 97, 98] Fan et al.[53, 57] reported that the visible emission in ZnO dendritic wires and nanosheets consisted of two components centered at 540 and 610 nm. The intense visible emission in ZnO nanosheets was tentatively attributed to surface dislocations.[57] Orange-red emission at 626 nm in ZnO nanorods was attributed to oxygen interstitials.[67] In addition, orange emission at 640–650 nm in ZnO needles[98] and nanowires[97] was proposed to be due to oxygen-rich samples, in agreement with a previous study on ZnO films.[99] This emission could be reduced by annealing under vacuum or in a H2/Ar mixture.[97] However, although these treatments quenched the visible defect emission, near-infrared (NIR) emission at 750 nm was enhanced.[97] It was shown that green, yellow, and red-NIR emissions originate from differ-

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ent types of defects by depth-resolved cathodoluminescence and PL measurements.[100] The NIR and the yellow emissions were found to have different decay properties, and it was proposed that they involved a similar final state related to excess oxygen but with different initial states (conduction band and donor centers).[101] It should be noted that although the majority of studies attribute red-NIR emission to excess oxygen, zinc interstitials were also proposed to explain the origin of a red emission in ZnO particles.[102] Thus, although this emission is less controversial than the green one, further studies are needed to clarify its origin. A similar conclusion applies to other defect emissions reported in ZnO, such as blue and violet defect emissions. Zhao et al.[55] reported emission at 3.0 eV (413 nm), which was attributed to a zinc vacancy, while a violet emission in ZnO nanobelts at 421 nm was attributed to interstitial zinc.[56] Blue emission at 440 nm was reported for tetrapodal nanocrystals,[59] while other reports indicate the presence of both violet (419 nm) and blue (438 nm) emissions in ZnO tetrapods.[60] The violet emission was attributed to interface traps, while blue emission was attributed to oxygen vacancies.[60] A blue emission band ( 420 and 444 nm) in ZnO nanowires[63] and ZnO nanocrystals (at 442 nm)[68] was also attributed to oxygen vacancies. It is obvious that for many types of defects, different studies assign them different emission-peak positions. Thus, while the origins of some of the emission peaks (such as yellow for example) are less doubtful, the origin of defect emissions in ZnO is still an unresolved question in spite of a large number of reports. In addition to identifying the origin of the defect emissions, an important question is the suppression of defect emission either by varying the fabrication conditions or by post-fabrication treatment. It was reported that the green emission from ZnO nanoparticles can be suppressed by embedding the nanoparticles into a synthetic opal whose photonic bandgap overlaps with the deep-level emission.[61] Another way to suppress green defect emission is by coating of the surface with surfactant.[36] Hydrogen plasma was also shown to enhance UV-to-defect emission intensity ratio for ZnO nanorods.[103] As for the yellow emission, it has been shown that it can be reduced by annealing in a reducing environment (hydrogen/argon mixture).[46]

2.3. Defect Identification using Other Techniques There are several techniques that can be used combined with photoluminescence to identify the origin of the defect emissions in ZnO. A useful technique for identifying paramagnetic defects is electron paramagnetic resonance (EPR) spectroscopy, which has been used to study defects in ZnO.[36, 87, 93, 95, 104, 126] For an overview of previous studies see Ref. [126], while some of the more recent results are given in Ref. [36]. Similar to the origin of the green luminescence, the assignment of the peaks in EPR spectra of ZnO has been controversial. The singly ionized oxygen vacancy hypothesis was proposed by Vanheusden et al.[92] based on the correlation between the EPR peak at g 1.96 and green emission intensity. However, other studies attribute the


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peak at g 1.99 to oxygen vacancies.[88] Although the assignment of the peaks is in question, this technique is still a useful tool to obtain more information about the defects, and its application to nanostructures is straightforward since nanostructures can be treated as any other sample in powdered form. Positron annihilation spectroscopy (PAS) has been used in the past together with photoluminescence to study defect emissions in ZnO.[105, 106] Although no conclusive identification of the defects responsible for the visible emissions has been made, this technique still enables more information to be obtainined about the defect levels, which helps to at least eliminate some of the possible candidates for the causes of defect emission. It should be noted however that although PAS has been applied to some nanostructured samples, the interpretation of the data from nanorods or nanowires would be very complicated due to the voids in the samples. Another useful technique for studying defect levels is deeplevel transient spectroscopy (DLTS). A DLTS study of ZnO single crystals established that defects at 0.10, 0.12, 0.29, and 0.59 eV below the conduction band were found,[107] although the defect identity was not conclusively established. Again, this technique is also more applicable to thin-film or singlecrystal samples. However, it is expected that conclusions from studying defect emission from ZnO in these forms could be extrapolated to identify defects in nanostructures since defect positions and behavior are similar for nanostructured and bulk samples.

Figure 9. Emission spectra from highly faceted ZnO rods at different excitation powers. The excitation wavelength was 267 nm, and the pulse duration was 1 ps.

significantly shorter decay time of the stimulated emission compared to spontaneous emission, lasing peaks can be observed more clearly in time-resolved spectra, as shown in Figure 10. As the excitation power increases, the increase in intensity and the appearance of narrow lasing modes can be observed. With a further increase of excitation power, lasing in the EHP mode occurs. The lasing in these different excitation regimes will be discussed below.

3. Stimulated Emission Due to its high exciton-binding energy, ZnO is of interest for the achievement of excitonic stimulated emission at room temperature, which has a lower threshold than electron–hole plasma recombination. While there have been numerous reports on optically pumped lasing and amplified spontaneous emission from ZnO,[4, 41, 98, 127–168] no electrically pumped lasing has been achieved as yet. Amplified spontaneous emission was reported for a self-organized network of ZnO fibers,[161] while lasing has been reported in a number of different structures such as, for example, nanowires,[5, 138] tetrapods,[151–154] and nanoribbons/combs.[154] A very broad range of lasing thresholds has been reported for different ZnO nanostructures, ranging from 8 kW cm2 (ZnO fibers)[130] to 867 kW cm2 (ZnO nanorods).[163] In the following section, the experimental results of the stimulated emission in ZnO nanostructures will be summarized. The discussion of the basic principles will include some comparisons with stimulated emission from other forms of ZnO.

Figure 10. Time-resolved PL spectra for spontaneous emission (shown at 4 ps), stimulated emission due to exciton–exciton scattering (shown at 8 ps due to a longer delay time), and stimulated emission due to EHP (shown at 4 ps). Due to the very high intensity emission in the EHP regime, the other two spectra have been multiplied by a factor of 10 to improve the clarity of presentation. Reprinted from Ref. [41].

3.1.1. Exciton–Exciton Scattering 3.1. Gain Mechanism

The peak position of the emission resulting from inelastic collisions between excitons is given by:[167]

Stimulated emission in ZnO can be achieved either by exciton–exciton (EE) scattering or electron–hole plasma (EHP) recombination. As the excitation power increases, sharp peaks will appear in the emission spectra from ZnO (highly-faceted rods), as illustrated in Figure 9. Due to the

En ¼ Eex Ebex 1 1 n2 3kT=2

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ð1Þ

where n = 2, 3,…, k is the Boltzmann constant, T is the temperature, and Ebex = 60 meV is the exciton binding energy.



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The transition from spontaneous emission to stimulated emission is evidenced by narrowing of the emission with a full-width half maximum (FWHM) about two orders of magnitude lower compared to the FWHM of spontaneous emission.[156] The threshold for lasing due to exciton–exciton scattering in nanostructures is typically 2–3 times lower[98, 138, 154] than that for lasing in the EHP regime. 3.1.2. Electron–Hole Plasma With increasing excitation energy, the density of the excitons in ZnO will also increase. As the exciton density increases, binding energy decreases. The EHP plasma forms at densities higher than the “Mott density”, given by:[1] nM ¼

kT 2a3B Ebex

ð2Þ

where aB is the Bohr radius. This is estimated to be 3.7 K 1019 cm3,[1] although lower estimates such as 4 K 1018 cm3 have also been reported.[138] The EHP emission is typically more broad and red shifted compared to emission due to exciton–exciton scattering.[1] The red shift of the EHP emission is the result of bandgap renormalization.[140] Coexistence of EE and EHP emissions observed in ZnO thin films was attributed to spatial nonuniformity of the sample and the beam profile.[127] However, time-resolved studies on different ZnO nanostructures indicate that the coexistence may originate from the fast decay of EHP emission since a blue shift of the emission with time and eventual bandgap recovery could be clearly observed in time-resolved spectra.[154] The evolution of the lasing spectra with an increase of excitation power has been studied in detail for single ZnO nanowires.[138]

Figure 11. Illustration of two different feedback mechanisms: a) Nanostructures as Fabry–Perot resonators, b) a random laser.

3.2. Feedback Mechanism The coherent feedback in ZnO nanostructures or nanostructured films can be provided by two basic mechanisms, as illustrated in Figure 11. In the first case, coherent feedback is provided by multiple reflections from the end facets of the nanostructure, which serves as a Fabry–Perot resonator. It has been shown that UV emission is typically enhanced at the ends of the nanowire, while defect (green) emission is emitted from all parts of the nanowire,[138] as shown in Figure 12. In the second case, the coherent feedback is provided by multiple scattering events. An example of random lasing from ZnO is illustrated in Figure 13. Random lasing and individual nanostructures as Fabry– Perot resonators are discussed in detail below. 3.2.1. Random Lasing In random lasers, coherent feedback is provided by recurring scattering events.[169] Cavities are “self-formed”, and the main requirement to observe this type of lasing is that the scatterer size is smaller than the emission wavelength.[162] The dependence of the lasing on the excitation

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Figure 12. Images of green/UV PL: a) Topographic, b) UV, and c) green near-field PL images. Scale bar = 5 mm. d) Far-field image of green/UV PL showing enhancement of the UV PL near the end of the wire. Scale bar = 5 mm. Reprinted with permission from Ref. [138].

area has been demonstrated in ZnO polycrystalline films,[128] nanorods,[133] and nanowires,[145] and the closed loops along which lasing occurred have been observed.[128] Random


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ferent nanostructures can have different lasing thresholds,[138] reduction in a lasing threshold observed with increased excitation area could simply be a result of an individual nanostructure with a lower threshold becoming excited. One possible way to distinguish between the two possible feedback mechanisms is to analyze the Fourier transform of the lasing spectrum.[145] In order to distinguish between the possibility of having multiple closed loops and multiple lasing modes in individual Fabry–Perot cavities formed by crystal facets of the nanostructrures, it may be necessary to observe where the lasing originates from, as described elsewhere.[128] Careful Figure 13. a) Schematic diagram of the laser measurement setup. b) Light curves for the samples after interpretation of the lasing various ion-irradiation times. The inset shows the maximum emission intensity of the TE mode as a funcdata is necessary to conclution of polarization angle. c) Emission spectrum of the as-grown ZnO thin film under a pump power of sively establish the feedback 2 1.6 MWcm . d) Evolution of emission spectra of the irradiated sample (30 min) under different pump mechanism. Although intensities. e) Emission spectrum of the sample irradiated for 60 min under a pump power of 2 random lasing has been pro1.6 MWcm . Reprinted with permission from Ref. [156]. posed to explain stimulated emission from short needles with sharp top surfaces,[156] lasing in ZnO was reported in polycrystalline thin [128, 143, 157, 159] [155, 160] [162] films, nanoparticles, ZnO powder films, stimulated emission was observed from individual, relatively short nanorods with pyramidal tops indicating that multiple nanorods,[133, 143] nanoneedles,[156] and nanowires.[145] The film scattering was not necessary to achieve stimulated emisstructure and crystallinity were found to affect the lasing sion.[166] threshold,[143] and the lasing characteristics were also found to be dependent on the strain.[157] Random lasing was also demonstrated from thin-film ZnO ridge waveguides, with[159] 3.2.2. Nanostructures as Fabry–Perot Resonators and without[159] a MgO capping layer. The lasing threshold Lasing from ZnO nanowires with each wire forming a in ZnO nanoneedle random lasers was found to be signifiFabry–Perot resonator bound by reflecting (0001) facets was cantly lower than the reported value for conventional lasing first reported by Huang et al.[4] Since then, there have been in ZnO nanocolumns.[155] numerous reports on stimulated emission from various ZnO One distinguishing characteristic of a random laser is nanostructures. Lasing was reported in microtubes,[129, 150] that stimulated emission can be observed in all directions, and the mode structure in the measured spectra shows angunanocoral reefs and nanofibers,[130] whiskers,[131] nanolar dependence.[162] While the interpretation of the data is wires,[4, 138, 139, 147–149] nanorods,[132, 134, 154] nanoribbons,[135, 136, 139] straightforward in the case of polycrystalline films, for the nanocombs,[137, 154] and tetrapod nanostructures.[151–154, 168] case of a nanowire ensemble with random nanowire orientaWhile some of the measurements have been performed on tion, it is difficult to establish from angular-dependence nanostructure ensembles, stimulated emission from individmeasurements of the emission spectra whether the feedback ual nanostructures[135, 136, 138, 139, 151, 152] as well as nanostructures originates from individual wires acting as resonators or from dispersed with very low density (< 10 per laser spot)[168] was scattering between the wires. Another distinguishing characalso obtained, clearly demonstrating that in those cases the teristic of a random laser is that the lasing threshold defeedback could not be obtained from multiple random scatpends on the excitation area.[162] The lasing threshold intering. While in some cases, such as individual nanowires[138] creases as the excitation area decreases, and eventually no and nanoribbons,[138, 139] the identification of the cavity is lasing can be observed in areas smaller than a critical straightforward, lasing has also been demonstrated with size.[162] However, careful analysis of the obtained data is nanostructures with more complex morphologies. Also, stimulated emission was obtained not only for nanostrucneeded when measurements are performed on an ensemble tures fabricated at high temperatures by vapor deposiof nanostructures instead of polycrystalline films. Since difsmall 2006, 2, No. 8-9, 944 – 961



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tion,[131, 137, 154, 168] but also for nanostructures fabricated at low temperature from aqueous solution.[134, 154] Detailed mode analysis of nanostructures as laser cavities has been performed.[138, 153] Good agreement in ZnO nanowires between the measured modes with similar polarization and expected mode spacing is given by:[138] Dl ¼

l2 dn 2L n l dl

ð3Þ

where Dl is the mode spacing, l is the wavelength, n is the refractive index, and L is the length. The linewidth of the measured lasing modes was also in good agreement from the theoretical estimate of the order of 1 nm obtained for a Fabry–Perot resonator by using the following expression:[138] c ln R1 R2 ð1 Ti Þ2 Dn ¼ 4pLn ð4Þ where c is the speed of light, R1 and R2 are the reflectivities of the mirrors, and Ti is the transmittance of the internal medium of the cavity. The threshold gain can be expressed as:[135] gth ¼ a þ

1 1 ln ðR1 R2 Þ 2L

Figure 14. ZnO tetrapod manipulation and lasing in various configurations. a) Spectra recorded at 91, 284, 468, and 738 mJ cm2, showing stimulated emission in the three-arms-down configuration. Inset (left): Power dependence curve showing a lasing threshold of 430 mJ cm2. Inset (right): PL image of the lasing tetrapod depicting the vertical arm as a bright spot in the middle of the structure. Scale bar = 5 mm. b) Spectra recorded at 255, 454, 596, and 766 mJ cm2 after flipping the tetrapod into the three-arms-up configuration. The lasing threshold for this geometry increased to 520 mJ cm2 and the mode shape was drastically altered in comparison to (a). Inset: PL image of the lasing tetrapod. Scale bar = 5 mm. c) SEM image of the same tetrapod in (a), (b), and (d) after removing one of the arms with a micromanipulator. Scale bar = 1 mm. d) Lasing spectra of the three-armed “tetrapod” shown in (c). The mode structure is similar to (b), but the lasing threshold increases further to 584 mJ cm2. Inset: PL image of the lasing tripod showing larger scattering loss from the left (top arm in the SEM image) damaged arm. Scale bar = 5 mm. Reprinted with permission.[153]

ð5Þ

where L is the length and a is the absorption loss. It should be noted, however, that thresholds in individual nanowires can vary by orders of magnitude, which was attributed to differences in dimensions, the condition of the cavity, and the extent of substrate coupling.[138] The threshold may also be affected by damage of the nanowire during transfer from the growth substrate to the support substrate for the measurements.[153] Among other more complicated cavities, ZnO tetrapods have been most thoroughly studied.[151–154, 168] The lasing threshold and mode structure of a ZnO tetrapod were found to be dependent on the tetrapod position on the substrate (resting on one arm or three arms), which indicated that the substrate coupling and the collection geometry played a role in the lasing spectra.[153] Intercavity coupling between different arms was also identified as a possible factor affecting the lasing threshold and the mode structure, although this effect is likely not very significant since removal of one tetrapod arm had only a small effect on the lasing properties, as shown in Figure 14.[153] This is in agreement with other reports which have shown that each leg of

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the tetrapod acts as an individual laser.[151, 152] The lasing from multiple tetrapod arms occurs only if all the arms are excited, otherwise lasing will occur only from the excited arm due to high losses in the non-excited tetrapod arms.[152]

3.3. Factors Affecting the Achievement of Stimulated Emission It has been pointed out that very high lasing thresholds are obtained in ZnO films with poor crystallinity.[143] Increase in the threshold with a decrease in crystal quality was attributed to an increased concentration of nonradiative defects.[143] On the other hand, lasing has been successfully obtained from nanorods grown by hydrothermal methods,[134, 154] which typically have inferior crystallinity to samples fabricated by vapor deposition due to the low synthesis temperature. The presence of strong defect emission does not prevent stimulated emission, since the UV-to-visible emission intensity ratio increases with increasing excitation power.[36] Also, the long decay time of spontaneous emission, which indicates excellent crystal quality, is not necessa-


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ry for the achievement of lasing. Biexponential decay with time constants of 70 and 350 ps was reported for ZnO nanowires with a lasing threshold of 40 kW cm2.[4] On the other hand, highly faceted rods with long luminescence decay times (116 ps and 1.2 ns) exhibited a lasing threshold of 45 mJ cm2 or 150 MW cm2.[41] It has been shown that stimulated emission can be achieved in nanostructures with different decay times and different defect emissions, but it could not be achieved if poor crystal quality and high cavity losses occur simultaneously.[170] The lasing threshold will therefore be mainly dependent on the dimensions of the nanostructure (see [Eq. (5)]), quality of the cavity, and experimental conditions. Also, the lasing thresholds obtained for different excitation wavelengths should not be directly compared due to different absorption of ZnO at different wavelengths. Due to great variations in the lasing thresholds of individual nanowires[138] and different experimental conditions, it is difficult to compare the lasing thresholds from different reports in the literature. Sometimes the reported results are contradictory. For example, it was reported that nanoribbons had a higher threshold compared to tetrapods and nanowires,[153] while nanoribbon/nanocomb mixtures synthesized by a different method exhibited a lower threshold than tetrapods and nanorods.[154] However, the fact that lasing was demonstrated in a great variety of nanostructure morphologies indicates that optically excited stimulated emission is easy to achieve in the majority of ZnO nanostructures. Improved crystallinity and larger dimensions can result in lower lasing thresholds, though some variation among nanostructures with similar dimensions fabricated in the same deposition procedure is expected due to different conditions of the cavity.

3.4. Time-Resolved Studies of Stimulated Emission Time-resolved studies of stimulated emission in ZnO have been performed by several groups.[41, 98, 127, 138–141, 151, 152, 154, 163, 168] The studies have been performed on ZnO thin films,[127, 140, 141] nanowires,[138, 139] nanoribbons,[139] highly faceted rods,[41] nanoneedles,[98] nanoribbons/combs,[154] tetrapods,[151, 152, 154, 168] and nanorods.[154, 163] Typically, the stimulated emission decay time is much faster than that of spontaneous emission, so that it may be below the detection limit of some time-resolved photoluminescence systems.[149] Emissions in the EE and EHP regime exhibit different behaviors with time.[41, 98, 154, 168] The comparison between the decay curves of the spontaneous emission, EE, and EHP emissions from highly faceted rods is shown in Figure 15. It is clear that although both types of stimulated emission have a shorter decay time compared to spontaneous emission, there are obvious differences in their temporal evolution. EHP emission typically has a short rise time (1–2 ps), which can be attributed to the thermalization of the hot carriers.[140, 141] The EHP emission peak exhibits some shifting with time, which was established by direct measurements of the lasing spectra[98, 140, 154, 168] as well as by measuring transient profiles of the lasing dynamics as a small 2006, 2, No. 8-9, 944 – 961

Figure 15. Decay curves for three different emission regimes. The inset shows an enlarged region in the range from 1 to 30 ps. Reprinted from Ref. [41].

function of wavelength.[152] The decay time of EHP emission is typically just a few picoseconds.[98, 151, 152, 154, 168] It was suggested that long cavity length, lower losses at end facets, and lower defect concentrations would result in longer decay times of the lasing.[152] Unlike EHP emission, stimulated emission in the EE regime can exhibit a longer delay time before the onset of the emission.[41, 139, 154, 168] This was attributed to the longer time needed to achieve a high concentration of excitons in the excited state.[139] Similar to EHP emission, decay time is also typically just a few picoseconds.[41, 154, 170] With respect to the evolution of the lasing spectra and any peak shifts in the EE regime, some spectral shifts of the peaks can be observed with time,[154] but it is difficult to analyze the data because the measurements have been performed on an ensemble of the nanostructures. It should also be noted that as a consequence of fast decay time of EHP emission and the longer delay time of EE emission, coexistence of the two can sometimes be observed in the time-resolved spectra obtained at different times.[154]

4. Nonlinear Optical Properties As a consequence of its non-centrosymmetric crystal structure, ZnO is expected to have nonzero second-order susceptibility. Nonlinear properties have been studied for different forms of ZnO.[171–189] Nonlinear optical response of C excitons has been measured by the four-wave mixing technique on ZnO single-crystal samples.[174] Second-harmonic generation (SHG) was measured in ZnO single crystals,[177] thin films,[171–174, 176, 178–185] nanowires,[139, 186] and nanoribbons.[139] SHG in ZnO thin films is dependent on the deposition conditions,[178, 180] film thickness,[171, 181] crystalline structure,[171, 179, 181] orientation of the crystallites,[182] and the grain shape.[184] The significant part of the SHG signal was found to be generated at grain boundaries and interfaces.[171] It was proposed that the film thickness more significantly affects the second-order susceptibilities than the deposition technique used.[183] Enhancement of the nonlinear suscepti-



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bility compared to the bulk values was obtained for very thin films.[172] The reduction of the second-order susceptibility with increasing film thickness was attributed to the change in orientation of the polar axis as film thickness increases.[183] It was also proposed that under certain experimental conditions, third-harmonic generation (THG) comparable to a conventional SHG signal can be observed in ZnO thin films.[186] Sputtered ZnO films exhibited both second- and third-order nonlinear properties in spite of the lack of a preferential growth direction, although the secondorder nonlinearities were lower than the bulk values.[173] However, THG has been less frequently studied[173, 174, 186] than SHG in ZnO. While a transient SHG technique was used to study carrier dynamics in ZnO nanoribons and nanowires,[139] and SHG and THG signals were measured in ZnO nanowires,[186] nonlinear optical properties of other ZnO nanostructures have not been studied. In addition to SHG and THG studies, two-photon[187, 188] and three-photon[187] spectroscopy and measurements of nonlinear refraction and absorption[188] of ZnO were also reported. Two-photon-induced photoluminescence was also reported in ZnO microtubes,[189] and two- and three-photon-induced luminescence was observed in single-crystalline ZnO.[175] However, multiphoton spectroscopy studies, as well as characterization of the nonlinear optical properties of nanostructured ZnO in general, have been scarce since both the measurement and the theory are more complex compared to the characterization of linear optical properties. Considering the variety of available morphologies and possible fabrication methods for ZnO nanostructures, it would be of particular interest if more studies of the nonlinear optical properties of different ZnO nanostructures were conducted.

5. Optical Properties of Doped ZnO The optical properties of doped ZnO have been widely studied.[190–228] In general, the effect of doping on the optical properties can be studied either by examining low-temperature UV spectra for evidence of the appearance of bound exciton peaks not observed in undoped samples, or by examining the visible emission for changes in the defect emission spectra. The doping of ZnO is a research topic of considerable interest in its own right, but the discussion will be limited here to the actual doping of nanostructures and nanocrystalline films. There are four main topics of interest in doping ZnO: 1) doping with donor impurities to achieve high n-type conductivity, 2) doping with acceptor impurities to achieve p-type conductivity, 3) doping with rare-earth elements to achieve desired optical properties, and 4) doping with transition metals to achieve desired magnetic properties. While doping to achieve n-type conductivity is straightforward, the achievement of p-type conductivity is difficult due to the presence of native defects. In addition to studying the effects of impurities on optical, electrical, and magnetic properties, the effects of different impurities on the orientation of ZnO nanorods fabricated by a combination

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of chemical vapor deposition and laser ablation were also studied.[199] It was found that some impurities (such as Er or Mn) can result in preferential orientation of the nanorods perpendicular to the substrate.[199] Doping is expected to induce some changes in the morphology of the nanostructures, although the effects have not been systematically investigated for the majority of dopants and growth methods. In addition to intentional doping, fabrication of hierarchical ZnO structures where other elements are present in the source material may result in the presence of secondary phases and the incorporation of impurities in ZnO, as in the case of Bi2O3-containing ZnO hierarchical nanostructures.[192] Group III and group IV elements are typically used to dope ZnO with donor impurities. Halogen atoms can also serve as donors in ZnO, and have the additional benefit of reducing oxygen adsorption on surfaces,[206] but they are less commonly used than group III dopants. Al-doped ZnO (AZO) films are commonly proposed as a transparent conductive oxide, which can replace for indium tin oxide electrodes in organic optoelectronic devices. AZO nanosheets and nanowalls have been reported.[201] It was found that the addition of Al2O3 to the source material resulted in Al doping, but the morphology changed from nanowires to nanosheets and nanowalls, while low-temperature cathodoluminescence spectra exhibited narrow donor bound-exciton lines, with the I6/8 line attributed to the Al donor,[201] in agreement with other reported assignments of this donor bound-exciton line.[76] Single-crystalline AlZnO nanowires/ nanotubes were also reported, and it was found that incorporation of Al resulted in an increased bandgap from 3.29 eV to 3.34 eV.[194] For the case of In doping, nanocrystalline films,[225] nanorods,[198] nanobelts,[222] and nanowires[210, 216] have been reported. ZnO and ZnO:In nanorods were fabricated from the reaction of zinc nitrate hydrate with hexamethylenetetramine (for In doping, indium chloride was added).[198] No significant differences in the morphology of the nanorods were observed after doping. A blue shift of the absorption peak and UV emission peak was observed, and the ratio of UV-to-green emission decreased after doping.[198] On the other hand, broadening and a red shift of the UV emission peak was observed in In:ZnO nanobelts, which was attributed to a significant increase in the carrier density.[222] A similar red shift of the UV emission peak due to heavy In doping was also observed in ZnO:In nanowires with a superlattice structure.[216] In general doping with different donors produces broadening of the UV emission peak, but the peak shift is dependent on the dopant.[210] It was found that Sn doping produces the largest red shift of the UV emission, as well as the appearance of strong green emission in doped ZnO nanowires.[210] On the other hand, another study of optical properties of Sn-doped ZnO nanowires reported no UV emission shift (380 nm) and the appearance of new emission peaks at 396, 461, and 502 nm.[219] Obviously, since both undoped and doped ZnO can exhibit different optical properties dependent on the fabrication conditions, it is difficult to establish how the properties will change after doping. The appear-


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ance of new luminescence peaks is expected, but in the case of significant increase in the carrier density, a red shift of the near band-edge emission is also expected. It was also reported that the optical properties of Sn-doped ZnO nanobelts were dependent on the type of flowing gas used, illustrating the importance of the fabrication conditions.[208] In addition to Sn, Pb represents another possible donor dopant,[219] and Pb-doped ZnO nanowires have been reported.[207] Pb doping was found to result in a red shift of the emission peak with the peak position dependent on Pb content.[207] Sc doping also resulted in red-shifting of the PL spectra with increasing concentration, remarkably similar to the results observed for Pb-doped ZnO nanowires.[213] Acceptor dopants in ZnO are usually group V elements, such as N, As, and P. Nitrogen doping was reported in ZnO crystals,[191] nanocrystals,[215] and nanorods.[204] N doping of ZnO crystals was found to result in the appearance of nitrogen-associated donor–acceptor emissions in low-temperature PL spectra.[191] While N-doped nanocrystals showed violet luminescence at room temperature (attributed to defect emission),[215] N-doped nanorods showed a strong UV emission at 3.31 eV and negligible defect emission.[209] In the case of As-doped ZnO nanowires, unusual behavior in temperature-dependent PL spectra has been observed, with acceptor bound-exciton emission detectable at room temperature.[203, 214] On the other hand, p-type P-doped ZnO films exhibited more normal behavior with the acceptor boundexciton peak dominant at low temperatures and free-exciton emission at room temperature.[190] ZnO nanostructures have also been doped with different rare-earth elements such as Tb,[228, 229] Ce,[217] Eu,[200, 204] and Dy.[193] In the case of Tb-doped ZnO nanoparticles, emission from both Tb and surface states was observed.[229] Tb emission increased with increasing Tb concentration while the emission from surface states decreased.[228] Ce incorporation into 1D ZnO nanostructures results in the appearance of a violet-blue emission and the disappearance of any green defect emission.[217] Eu-related emission was observed from ZnO:Eu nanorods for a suitable chosen excitation wavelength.[200, 204] On the other hand, Dy-doped ZnO nanowires exhibited only a UV emission of ZnO with only a very weak emission attributed to Dy.[193] Therefore, when doping with rare-earth elements, it is possible that their emission will be masked by ZnO defect emission, and thus the excitation wavelength must be carefully chosen to establish the effects of doping on optical properties. Doping with transition metals is expected to result in changes in the magnetic properties of the material, and Mndoped ZnO has been predicted to be ferromagnetic at room temperature. Therefore, there has been considerable interest in the fabrication and characterization of transition-metaldoped ZnO nanostructures. Mn-doped nanocrystalline films,[227] tubes,[195] nanorods,[197] multileg nanostructures,[211] nanobelts,[224] and tetrapods[223] have been reported. Other dopants include Ni (nanowire arrays)[196] and Co (nanocluster films).[202] Mn-doped ZnO rods were found to be ferromagnetic at room temperature,[197] but both the magnetic and optical properties of the Mn-doped nanostructures are strongly dependent on the fabrication conditions. Mn small 2006, 2, No. 8-9, 944 – 961

doping was found to quench green emission,[227] although other studies reported reduction in both UV and defect emission.[195] Decrease in UV emission and the appearance of green emission after Mn incorporation have also been reported.[197] In addition, a blue shift and increase in intensity of the UV peak were found after Mn doping.[211] Very similar spectra of ZnO and Mn-implanted ZnO were observed after annealing an implanted sample at 800 8C.[224] A similar UV-to-green emission ratio has been observed in undoped and Mn-doped ZnO tetrapods.[223] Obviously, the change in the optical properties is strongly dependent on the method of incorporation of Mn, fabrication conditions, and properties of undoped ZnO fabricated under similar conditions. In the case of Ni doping, a red shift of the UV emission with no significant change in the visible part of the spectrum was observed.[196] Co doping also resulted in a small red shift of the UV peak, as well as peak broadening.[202] Other dopants whose effects on the optical properties of ZnO nanostructures have been studied include sulfur[205, 212, 221, 226] and copper.[218, 220] Enhancement of green emission with S doping has been reported,[205, 221] as well as the change in shape of the broad green defect emission.[212] Either no significant shift[221] or, in contrast, a blue shift of the UV emission peak[205, 212, 226] has been reported. In case of Cu doping, broad PL spectra extending from the UV to the red spectral region were observed in Cu:ZnO nanowires.[220] However, Cu-doped ZnO nanowires prepared by a different method only showed an increased red shift of the emission peak with increasing copper content.[218] Therefore, it can be concluded that regardless of the type of dopant, the optical properties of the nanostructures have a very strong dependence on fabrication conditions. Thus, very careful interpretation of the measured spectra of doped ZnO nanostructures is necessary.

6. Conclusions and Outlook A great variety of ZnO nanostructures have been reported. Their optical properties have been studied mostly at room temperature, although variable-temperature photoluminescence studies on different ZnO nanostructures have now been reported. Similar to other forms of ZnO, the origin of different defect luminescence peaks remains unresolved. In general, the optical properties of various nanostructures are very similar to those reported for thin films, and the observed size effects in nanorods and nanowires are the result of different surface-to-volume ratios rather than through quantum confinement. All of the defect emissions reported in nanostructures can also be found in thin-film or bulk ZnO samples. The least-controversial defect emission is the broad yellow luminescence commonly observed in samples prepared from aqueous solutions, which likely originates from transitions involving interstitial oxygen; the origin of other defect emissions require further study. Although several methods for reducing or eliminating defect emissions, such as annealing in an Ar/H2 mixture for yellow emission and surface functionalization and hydrogen treatments for green emission, comprehensive study on the



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relationship between fabrication conditions and visible emission spectra is still needed. In order to conclusively identify the nature of defects, it will likely be necessary to combine PL studies with other experimental techniques such as EPR, carrier-concentration determination, and Xray photoelectron spectroscopy (XPS). In particular, annealing studies in different atmospheres and at different temperatures may be a useful tool in establishing the origin of the defect emission since annealing may result in the change of optical properties without changing the morphology. As for the stimulated emission, while ultrafast carrier dynamics has been well studied in the EHP regime, further studies are needed for lasing due to exciton–exciton scattering, which is of higher practical interest. Also, for practical applications of ZnO lasers it is necessary to achieve electrically pumped lasing in ZnO. Closely related to this issue is the achievement of reliable p-type doping with high carrier concentrations and mobilities, which is difficult due to the presence of native defects in ZnO. Although several groups have reported p-type doping in ZnO, this issue is still under scrutiny.[1] Research on doped ZnO nanostructures has been scarce compared to research on the doping of thin films and on undoped nanostructures. Finally, while there are numerous reports on the linear optical properties of ZnO nanostructures, much work remains to be done on studying their nonlinear properties.

Acknowledgements This work is partly supported by the Research Grant Council of the Hong Kong Special Administrative Region, China (HKU 7019/04P) and a University Development Fund grant of the University of Hong Kong.

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