OneDimensional CdS Nanostructures: A Promising ...

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Huiqiao Li, Xi Wang, Junqi Xu, Qi Zhang, Yoshio Bando, Dmitri Golberg, Ying Ma, and Tianyou Zhai* nanoscale electronic and optoelectronic devices.[8–13] Nanocircuits built using semiconductor nanowires were declared a “breakthrough in science” by the journal Science in 2001.[14] In addition, Nature published a report claiming that “Nanowires, nanorods or nanowhiskers. It doesn’t matter what you call them, they’re the hottest property in nanotechnology”.[15] 1D nanostructures are beyond doubt the hottest field in nanoscience and nanotechnology.[2,16] Based on the data from the Institute of Scientific Information (ISI), since 2000, the publications related to 1D nanostructures-related topics has exceeded the number of 158 000.[17] Recently, a book has been published titled One-Dimensional Nanostructures: Principles and Applications, in which 68 internationally renowned experts in this field (including the present authors) from 11 countries of 4 continents have been assembled to contribute chapters covering a broad overview in important 1D nanostructure topics.[2] As a vital and classical II–VI semiconductor, cadmium sulfide (CdS) is a fascinating material with versatile and fundamental properties such as the direct bandgap, relative low work function, high refraction index, excellent transport properties, good chemical and thermal stability, high electronic mobility, and piezoelectricity.[18–22] These unique properties make CdS one of the most important electronic and optoelectronic materials with prominent high-technology applications in nonlinear optical devices, flat panel displays, light-emitting diodes (LEDs), filed-emitters, photodetectors, waveguides, sensors, transistors, energy harvesting devices including photovoltaics, photoelectrochemical cells and nanogenerators.[23–26] The research on 1D CdS nanostructures has received worldwide concerns from nanoscience and nanotechnology experts, which has led to the notable increase of the numbers of research papers and patents related to them.[27] Herein, we highlight a selection of important topics pertinent to optoelectronical applications of 1D CdS nanostructures over the last 5 years. We begin with the rational design and controlled synthesis of CdS nanostructure arrays, alloyed nanostructucures and kinked nanowire superstructures, and then mainly focus on the optoelectronical properties and applications of 1D CdS nanostructures, as shown in Figure 1. Finally, the general challenges and the potential future directions of this exciting area of research are envisioned.

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One-Dimensional CdS Nanostructures: A Promising Candidate for Optoelectronics

As a promising candidate for optoelectronics, one-dimensional CdS nanostructures have drawn great scientific and technical interest due to their interesting fundamental properties and possibilities of utilization in novel promising optoelectronical devices with augmented performance and functionalities. This progress report highlights a selection of important topics pertinent to optoelectronical applications of one-dimensional CdS nanostructures over the last five years. This article begins with the description of rational design and controlled synthesis of CdS nanostructure arrays, alloyed nanostructucures and kinked nanowire superstructures, and then focuses on the optoelectronical properties, and applications including cathodoluminescence, lasers, light-emitting diodes, waveguides, field emitters, logic circuits, memory devices, photodetectors, gas sensors, photovoltaics and photoelectrochemistry. Finally, the general challenges and the potential future directions of this exciting area of research are highlighted.

1. Introduction Since the identification of carbon nanotubes in 1991,[1] onedimensional (1D) nanostructures with various shapes and morphologies, such as nanowires (NWs), nanotubes (NTs), nanorods (NRs), nanobelts/nanoribbons/nanosheets (NBs/ NRBs/NSs), nanocables and heterostructures, have attracted tremendous attention due to their significance in basic scientific research and potential technological applications based on their specific geometries and distinct properties.[2–7] These materials are regarded as the most ideal systems to investigate the dependence of electrical, optical, or mechanical properties on dimensionality and size reduction. They are also expected to be the most promising building block for the next-generation

Dr. H. Q. Li, Dr. J. Q. Xu, Dr. Q. Zhang, Prof. Y. Ma, Prof. T. Y. Zhai State Key Laboratory of Material Processing and Die & Mould Technology School of Materials Science and Engineering Huazhong University of Science and Technology (HUST) Wuhan 430074, P. R. China E-mail: [email protected] Dr. X. Wang, Prof. Y. Bando, Prof. D. Golberg International Center for Materials Nanoarchitectonics (WPI-MANA) National Institute for Materials Science (NIMS) Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan

DOI: 10.1002/adma.201300244

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2. Nanostructure Growth 2.1. CdS Nanostructure Arrays Semiconductor nanostructure arrays have drawn great scientific and technical interest due to their strong non-linear and electrooptical effects and promising applications in lasers, LEDs and field-emitters.[28,29] Aligned growth of nanostructures can be achieved with the use of templates, substrates and catalysts, or seeds. We fabricated morphology-tunable ordered CdS nanostructure arrays by a two-step metal–organic chemical vapor deposition (MOCVD) process using Au nanocrystals as catalysts.[30] The CdS ordered arrays uniformly and compactly covered the entire substrate, and the morphologies and alignments of nanostructure arrays were quite different for variable distances between the precursors and substrates. With increasing the distances, the morphologies varied from truncated nanocone arrays, NR arrays, cleft NR arrays (as shown in Figure 2a), and NW arrays. These 1D CdS nanostructures of various types displayed notable differences in stimulated and field emission performances. Very recently, Xiong and co-workers developed a general strategy for achieving aligned CdS NW and NB arrays utilizing van der Walls epitaxy with (001) muscovite mica substrate, without the aid of foreign metal catalysts.[31] These nanostructures exhibited uniform diameter through their length, sharp interface with the substrate (Figure 2b), and positive correlation between diameter and length, and the preferential growth direction of [0001]. Template-assisted synthesis is one the most elegant methods to prepare NR and/or NT arrays.[34] The template can be porous membranes including aluminum oxide, polycarbonate,

Tianyou Zhai received his Ph.D. degree from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2008. Afterwards he joined in National Institute for Materials Science (NIMS) as a JSPS Postdoctoral Fellow and then as an ICYS-MANA Researcher. Currently, he is a Full Professor at the School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), P. R. China. His research interests include the controlled synthesis and exploration of fundamental physical properties of inorganic functional nanomaterials, as well as their promising applications in energy science, electronics and optoelectronics.

mesoporous silica and removable NWs/NTs.[35] Well-aligned CdS NRs were synthesized within the nanochannels of porous anodic alumina membranes (AAMs) by thermal evaporation of CdS powders using Au seeds as the catalysts by Fan and co-workers.[32] The AAM was partially and controllably etched in a NaOH solution which was highly selective and did not chemically react with the CdS NRs. Figure 2c shows the CdS NR arrays with an exposed length of H ∼ 500 nm. The exposed depth was varied by tuning the etching time. Song and co-workers developed a layer-by-layer deposition approach to fabricate CdS NT arrays using ZnO NR arrays as removable templates.[33] The wall thickness of CdS NTs were precisely tuned by controlling the cycles of layer-by-layer deposition (Figure 2d).

2.2. Ternary CdS Nanostructures

Figure 1. Schematic diagram showing the potential opto-electronic properties and applications of typical one-dimensional CdS nanostructucures (acronyms: NW: nanowire; NR: nanorod, NB: nanobelt, NT: nanotube) such as cathodoluminescence, lasers, light-emitting diodes (LEDs), waveguides, field emitters, field-effect transistors (FETs, including logic circuits and memory devices), photodetectors, gas sensors, photoelectrochemistry, and photovoltaics, etc.

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Another important task in nanostructure growth is the accurate control and full-range tunability over the composition of alloyed and doped nanostructures, which are of particular importance for opto-electronic applications due to their tunable emissions.[36,37] CdS can form a series of alloyed materials, such as CdxZn1–xS and CdSxSe1–x, allowing continuous variation of the bandgap and thus subsequently the emission wavelength through changing their constituent stoichiometries.[38,39] Lee and co-workers fabricated CdxZn1–xS NRBs of variable compositions (0 ≤ x ≤1) by combiningg laser ablation of CdS with thermal evaporation of ZnS at 950 °C.[43] These NRBs had a thickness of 50–80 nm, a width of 0.5–5.0 μm, and


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Figure 2. SEM images of (a) CdS NR arrays synthesized by a two-step metal-organic chemical vapor deposition (MOCVD) process using Au nanocrystals as catalysts. Reproduced with permission.[30] Copyright 2009, American Chemical Society; (b) CdS NW and NR arrays grown on muscovite mica substrates through a hydrogen-assisted thermal evaporation. Reproduced with permission.[31] Copyright 2011, American Chemical Society; (c) CdS NR arrays fabricated by a template-assisted thermal evaporation process. Reproduced with permission.[32] Copyright 2009, Macmillan Publishers; (d) CdS NT arrays fabricated through the layer-by-layer deposition using ZnO NR arrays as removable templates. Reproduced with permission.[33] Copyright 2009, Springer.

a length of up to several hundred micrometers (Figure 3a). Their morphology slightly varied with a substrate temperature, whereas the composition was highly dependent on this parameter. These CdxZn1–xS NRBs possessed sharp, tunable lasing emissions within 340–390 nm and 485–515 nm. Jiang and co-workers achieved CdSxSe1–x NBs (see Figure 3b) with controllable compositions via a two-step sulfurization method though thermal annealing of CdSe NBs in a H2S-Ar atmosphere.[44] Such alloyed NBs exhibited wavelength tunable sharp near band gap emission and lasing action shifting continuously from 542 nm (x = 0.82) to 668 nm (x = 0.12). Recently, Sow and co-workers have developed a simple onestep approach with a specially designed substrate holder to synthesize ternary CdSxSe1–x NBs with uniform stoichiometries and accurately controllable compositions (0 ≤ x ≤1).[40] Photoluminescence measurements of CdSxSe1–x NBs revealed the emission with a tunable wavelength from 507 nm (pure CdS) to 713 nm (pure CdSe). The above-mentioned process of fabricating alloyed nanostructures using ME powders (CdS, ZnS or CdSe) with a high melting point (such as 1750 °C for CdS) involved vapor generation, transport and deposition of target materials, and inevitably required high temperatures or high-vacuum laser-ablation operations to generate sufficient amounts of vapor for later deposition. To resolve these problems, we developed a low-temperature route for the preparation of single-crystalline ternary CdxZn1–xS nanocombs and

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Figure 3. (a) SEM image of CdxZn1–xS NRBs fabricated by combining laser ablation of CdS with thermal evaporation of ZnS. Reproduced with permission.[43] Copyright 2005, Wiley-VCH; (b) SEM image of CdSxSe1–x NBs synthesized via sulfuring the CdSe NBs. Reproduced with permission.[44] Copyright 2009, American Chemical Society.

zigzag NWs via a one-step MOCVD approach by heating a mixture of Zn(S2CNEt2)2 and Cd(S2CNEt2)2 powders,[41,42] which reduced the growth temperatures to 400–420 °C. Their constituent stoichiometries were tuned by changing the relative proportion of the precursors. 2.3. Kinked CdS NW Superstructures Hierarchical nanostructures with modulated compositions, structures and interfaces have recently become of particular interest with respect to potential applications in nanoscale building blocks of future optoelectronic devices and systems.[46] Tian and co-workers developed a “nanotectonic” approach that provides iterative control of the nucleation and growth of the NWs,[45] and employed this strategy to grow Si, Ge and CdS kinked NWs for which the straight sections were separated by triangular joints with one fixed at 120°, as shown Figure 4a. The kinked NW growth involved three main steps during the nanocluster-catalyzed growth: axial growth of NW arm segment, purging of gaseous reactants to suspend NW elongation, and supersaturation and nucleation of NW growth following by the re-introduction of reactants. In the kinked NW synthesis, the length and number of straight segments were well controlled by the growth time and the growth



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and field effect transistors, with specific device functions precisely localized at the kinked junction in the NWs.[45] Such ideally designed and self-labeled NW structures should initiate unique directions and applications in the bottom-up integration of active devices.[47]

3. Opto-Electronic Properties and Applications 3.1. Cathodoluminescence

Figure 4. CdS kinked NWs: (a) Schematic of a coherently kinked NW and the secondary built unit (SBU) containing of two straight arms connected by one fixed 120° angle joint; (b, c) SEM and TEM images of one CdS kinked NW. Reproduced with permission.[45] Copyright 2009, Macmillan Publishers.

pressure variation, respectively, while the crystallographic growth direction was preserved. CdS kinked NWs were grown in a three-zone furnace by thermal evaporating CdS powder at 650–720 °C, with NW growth using Au nanoclustercatalyzed VLS process at 500–550 °C. These CdS NWs exhibited well-defined kinks with an angle of 120° (Figure 4b), and single-crystalline structure with arms, all along the direction of the wurtzite phase (Figure 4c). The authors also realized dopant-modulated structures, including p-n diodes

Cathodoluminescence (CL) is a useful technique for characterization of nanostructure optical properties due to its high spatial resolution and structural information obtained by using secondary electron (SE) imaging.[48,49] We first investigated the size-dependent CL properties on individual CdS micro/nanostructures, and provided the direct evidence for the origin of defect-level (DL) emission in such structures.[50] The CdS nanotips were composed of branched nanostructures with needleshaped ends (Figure 5b), and exhibited various defects along the nanotip. Within them, the defect concentration decreased towards the tip-end (Figure 5c). In order to study spatial variation of optical properties and to gain an insight into the origin of the DL emission, we recorded CL spectra at spots with different diameters along an individual nanotip. The CL spectra consisted of sharp and week green near-band-edge (NBE) emission peaks located at 530 nm, and broad and strong red DL emissions centered at 725 nm (Figure 5a). Furthermore, the intensity of DL emission (IDL) decreased with diameter shrinking in the normalized CL spectra compared with the NBE emission. The intensity ratio of DL to NBE emissions (IDL/INBE) also decreased with a diameter decrease, indicating that the NBE emission became dominant at spots within smaller diameter parts. Supposing that recombination centers

Figure 5. (a) Cathodoluminescence spectra recorded in different regions of the CdS nanotip in b; (b) SEM image of a CdS nanotip; (c) HRTEM images recorded in different regions of the nanotip in b. Reproduced with permission.[50] Copyright 2009, Wiley-VCH.

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NW lasing under optical pumping was first observed in ZnO NW arrays by Yang’s group,[52] and single NW lasing behaviors under optical and electrical pumping in CdS NW was first detected by Lieber’s group.[53] So far, optically pumped coherent laser emission of 1D CdS nanostructures has been well investigated, and various morphologies of CdS-based nanostructures have been found to support the lasing, including NWs,[54–56] NRs,[30] NBs,[57,58] single NW and NB,[59–62] ZnCdS NBs,[63] and CdSeS NWs.[64] Well-faceted NWs and NRBs are formed in both linear structures (Fabry-Pérot, F-P, cavity) and rings Figure 6. (a) Schematic of CL measurements on a bent CdS NW with a rectangular cross- (ring cavity); the ring cavity usually offers section; (b) The strain distribution in the cross-section; (c, d) SEM image and CL spectra of better resonance due to recirculation of light a bent CdS NW. The color of each spectrum in (d) corresponds to that of the circles in (c). in a ring. Tong and co-workers demonstrated [ 51 ] Reproduced with permission. Copyright 2011, Springer. a pigtailed NRB ring laser constructed of a 600 nm wide and 330 nm thick NRB.[62] The were only surface-related or distributed uniformly at the nanoNRBs were dispersed on an MgF2 substrate, and then assemtips, IDL/INBE should increase with decreasing diameter or be bled into a 20 μm diameter ring structure by circularly folding of the NRB in a side-by-side geometry, as shown in Figure 7a. independent of it. This was obviously opposite to our experiWhen the ring was irradiated by light from a supercontinuum mental observations. Consequently, as the defects were responsource, multi-longitudinal mode laser emission was observed at sible for the DL emission, we presume that the origin of DL around 523.5 nm with a full width at a half maximum (FWHM) was related to the peculiar defect distribution in the nanotip of 0.27 nm and a Q factor (Q = λ/Δλ) of 1900 (Figure 7b). bodies, as verified by HRTEM results (Figure 5c). The defect concentrations decreased with diameter shrinking, thus the DL The laser output from the pigtail showed strong orientationemissions also diminished.[50] dependent polarization, with a maximum polarization ratio of 5 and a power up to 3.5 nW.[62] Very recently, the same authors Yu and co-workers systematically investigated the straingradient induced exciton energy in curved CdS NW by optical further achieved a single-mode NW laser with low threshold CL measurements at a low temperature.[51] The CL experithrough folding one or two ends of a NW into microloops to form loop mirrors and coupled resonant cavities.[65] The reflecmental process and strain state of curved CdS NWs are shown in Figure 6a and Figure 6b. The CL spectra were collected tivity of a loop mirror was much higher than that of an endface when the electron beam was focused on the CdS NWs. Typical of the NW, resulting in the low threshold of the NW laser. The CL spectra acquired at different positions along the curved loop structure could tune the NW cavity, thus making an imporCdS NWs are shown in Figure 6d (the color in Figure 6d cortant step towards the realization of a tunable single–mode NW responds to that of the circles in Figure 6c). At the strain free laser.[65] It is known that the wavelength of a semiconductor part, the NBE peak was observed at 2.52 eV. When entering in laser is determined by its fundamental band gap, thus the bandthe curved region, a red-shift of the NBE peak occurred and gap-tunable alloyed semiconductors can be used to realize the increased with increasing the strain gradient. Moving beyond wavelength-flexible lasers through changing their constituent the most bent region, the red-shift decreased; moving to the stoichiometries. Tunable lasing has been demonstrated in other strain free part, the NBE peak occurred at the original alloyed CdSeS NWs at low temperatures.[64] By controlling the level. The largest red-shift of the NBE peak reached a value of substrate temperatures in a chemical vapor deposition system, 32 meV, where the strain gradient was 0.7 μm−1. The bending Ning and co-workers achieved spatial composition grading of ternary CdSeS NWs on a single substrate. Optical excitation of deformation-induced red-shift showed a linear correlation CdSeS NWs yield lasing with a tunable wavelength of 503 nm with the strain-gradient in the curved CdS NWs. The authors to 692 nm (Figure 7c). Similarly, tunable lasing has also been also designed a model of the curved CdS NWs and performed observed in ZnCdS NRBs.[63] refined density functional calculations, and revealed that at the density functional theory level a bending-induced red-shift Laser technology has made great progress towards higher indeed occurred in the nanosystem. Such linear strain-gradient power, faster, and smaller light sources. However, both optical effect on the band gap of semiconductors would open up a new mode size and physical device dimension of the traditional avenue in nanoelectronics and NW-based flexible devices.[51] lasers are restricted to a half of the the wavelength of the optical field due to the diffraction limit of light.[66] In 2009, Zhang and co-workers created the world’s smallest semiconductor laser, 3.2. Lasers i.e., nanometer-scale plasmonic laser, capable of generating visible light far below the diffraction limit.[67] They realized such plasmonic lasers based on the hybrid plasmon-mode con1D nanostructures are ideal miniaturized laser light sources cept by placing a CdS NW atop a Ag substrate separated by a due to their 1D geometry, dislocation-free single-crystalline nanometer-scale MgF2 gap, as shown in Figure 8a. The device nature, high index of refraction and atomically smooth surface.

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Figure 7. (a, b) Schematic and lasing spectrum from an individual CdS NRB ring, and integrated emission power versus pump energy (inset of (b)). Reproduced with permission.[62] Copyright 2010, American Institute of Physics. (c) Lasing spectra from CdSSe NWs with Se composition increasing towards the right. Reproduced with permission.[64] Copyright 2009, American Chemical Society.

was sealed in a vacuum chamber cooled to the cryogenic temperature (T < 10 K). Under the control of this unique hybrid design, the light can be amplified. Upon increasing the pump intensity, the onset of the amplified spontaneous emission peaks becomes apparent (Figure 8b). These correspond to longitudinal cavity modes that form when propagation losses are compensated by gain amplification, allowing plasmonic modes to resonate between the reflective NW end facets. Increasing the pump intensity further produces sharp lasing peaks. The clear signature of multiple cavity mode resonances at higher pump powers demonstrates sufficient material gain to achieve full laser oscillation, shown by the nonlinear response of the integrated output power with increasing input intensity

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Figure 8. Deep subwavelength plasmonic laser: (a) Schematic diagram and SEM image of the plasmonic laser consisting of a CdS NW (d means diameter) on top of a Ag substrate, separated by a nanometer-scale MgF2 layer (h means thickness); (b) Laser oscillation of a plasmonic laser (d = 129 nm, h = 5 nm). The four spectra for different peak pump intensities exemplify the transition from spontaneous emission to amplified spontaneous emission to full laser oscillation. Insets show the microscope images of the laser output; (c) Nonlinear response of the output power to the peak pump intensity. Reproduced with permission.[67] Copyright 2009, Macmillan Publishers.

(Figure 8c). The lasing light exciting from the NW end facets was easily detected in a far-field microscope image (insets of Figure 8b). It is possible to downscale the dimensions of both the devices and the optical modes due to the plasmonic modes without cutoff.[67] This achievement can enable the development of many innovations, such as single-molecule biodetectors, high-speed optics-based telecommunication systems, and photonic circuits. To make these plasmonic lasers usable for practical applications, it is needed to find a way to operate them at room temperature. Thanks to the great contribution of Zhang and co-workers, a semiconductor plasmonic laser work operating at room temperature was developed in 2011.[68] A 45-nm-thick CdS nanosquare is setting atop a Ag surface separated with a 5-nmthick MgF2 gap, as shown in Figure 9a. The surface plasmons of this system carry high momentum, even higher than light waves in bulk CdS and plasmonic NW lasers, leading to a strong


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3.3. Light-Emitting Diodes (LEDs)

Figure 9. Room-temperature plasmonic laser: (a) Schematic diagram of the room-temperature plasmonic laser showing a thin CdS square atop a Ag substrate separated by a 5 nm MgF2 gap; (b, c) Room-temperature laser spectra, integrated light-pump response (inset, up) and SEM image (inset, down) of multi-mode and single-mode plasmonic lasers. Reproduced with permission.[68] Copyright 2009, Macmillan Publishers.

feedback by total internal reflection of surface plasmons at the cavity boundaries. The close proximity of the high-permittivity CdS nanosquare and Ag surface enables modes of CdS square to mix with surface plasmon polaritons (SPPs) of the metaldielectric interface, leading to strong confinement of light in the gap region with a relatively low metal loss. For larger pump intensities, multiple cavity modes appear with orders of

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magnitude-higher coherence than the underlying spontaneous emission, as shown in Figure 9b. This laser was considered to be a SPASER (surface plasmons amplified by stimulated emission),[69] due to generating plasmonic cavity eigenmodes and emitting the light to the far-field as a side-effect of scattering. Single-mode plasmon lasing was also observed in an irregular shaped device with low symmetry where only a limited number of modes can undergo total internal reflection (Figure 9c).[68]

With the increasingly high demands of a green and energysaving world, there is a growing tendency to replace the conventional light sources including the fluorescent, incandescent, and high-pressure discharge lamps with LEDs. NW LEDs are usually fabricated using both inter-NW p-n junctions formed at the crossing points of p- and n- type NWs, and intra-NW p-n junctions formed by axial or radial (core-shell) modulation doping. Lieber and co-workers assembled p-n junctions using p-Si NWs (Figure 10a) and a variety of other n-type NW materials, including CdS, CdSeS, CdSe and InP to produce multicolor, electrically driven nanophotonic and integrated nanoelectronicphotonic systems.[70] As shown in the inset of Figure 10b, the I–V curve recorded from a p-Si/n-CdS junction shows clear current rectification. Under a forward bias voltage of >2.6 V, the CdS-Si crossed junctions yielded light emission of 510 nm. However, these crossed NW LEDs exhibited small effective heterojunction areas, which resulted in the low quantum efficiency (0.1–1%).[70] Later, the core/shell heterostructured NWs fabricated by multi-steps reactions were constructed as device elements to improve the device efficiency through increasing the heterojunction areas (Figure 10c);[71,72] however, the synthesis followed by device fabrication was expensive, complicated and time-consuming. Recently, Dai and co-workers developed highefficiency parallel-NBs heterojunction LEDs (PNBs-HLEDs) based on n-CdSxSe1–x NBs and p-Si NBs; this not only improved the device efficiency but also reduced the fabrication cost.[73] These two NBs forming the heterojunction were face-contacted and parallel to each other (Figure 10d). Compare with the “point-contacted” devices of crossed p-n NWs, this device structure has the larger active region and smaller series resistance, resulting in the higher electron and hole injective current and low turn-on voltage. The n-CdSxSe1–x/p-Si PNBs-HLEDs were fabricated as follows. First, the CdSxSe1–x NBs suspension was dropped onto the silicon-on-insulator (SOI) substrate (100 nm p-Si on 380 nm insulator) and photoresist pads were patterned by UV lithography on the SOI substrate. Second, the Si was etched by inductively coupled plasma (ICP) etching using the photoresist pads and NBs as masks. Finally, the remaining photoresist was removed, and the designed metal electrodes were formed on the NB. This process was simple and only used UV lithography and ICP etching process. Furthermore, the CdSxSe1–x NBs had tunable light emission from green (510 nm for CdS) to red (708 nm for CdSe) by changing the x value from 1 to 0. Under a forward voltage, light spots with a wide range of color were seen from the exposed ends of NBs in PNBs-HLEDs (up, Figure 7d). This approach of fabricating PNBs-HLEDs can be easily extended to other material systems.[73]



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spots along the CdS NB. With increasing the propagation distances from zero to 300 μm, the emission bands exhibited an apparent red-shift (from 517 nm to 531 nm), and the PL intensity decreased to one percent of the initial level (the mean optical loss was about 0.026 dB/μm; while under further increasing the distances (>300 μm) the emission band and intensity were kept stable. The authors proposed a band-tail mechanism to well describe this waveguide behavior in CdS NRB. The band-edge emission after laser excitation were re-absorbed by the band-tails in the bandgap and re-emitted with a lower energy photon. This absorption–emisison– absoprtion (A-E-A) process reduces some loss of energy, resulting in the red-shift of the PL band. Once the energy of the traveled light is smaller than that of the lowest side of band-tail, the A-E-A process breaks down and light propagates passively via electromagnetic waves with almost no lost photon energy.[77] Figure 10. NW LED: (a) Schematic of a n-CdS/p-Si crossed NW LED; (b) EL spectra from Recently, the same authors further fabrin-CdS/p-Si and n-CdSSe/p-Si crossed NW LEDs. Left-hand-side insets are the corresponding cated the CdS/CdS:SnS2 superlattice wires EL images, and the top-right inset shows I–V and SEM data recorded from a p-Si/n-CdS crossed through a co-evaporation technique with [ 70 ] NW LED (scale bar = 1 μm). Reproduced with permission. (c) Schematic of a single coreshell Si-CdS heterostructure NW LED. Reproduced with permission.[72] (d) Schematic and EL local environmental control and investigated [78] Such novel images of color tunable n-CdSxSe1–x/p-Si parallel-NBs heterojunction LEDs. Reproduced with their waveguide behaviors. [ 73 ] superlattice wires with diameters ranging permission. Copyright 2010, Royal Society of Chemistry. from 400 nm to 2 μm can modulate exciton 3.4. Waveguides emission and photon propagation with spectral periodical multipeaks. Figure 11a shows a PL image with well-defined emission The development of 1D nanostructure waveguides is an important step towards future nanoscale photonics since they can interconnect different types of electronic elements to carry out complex tasks such as logic operations, information storage and advanced communications.[74] CdS NWs with near-cylindrical geometry and large dielectric constants are the ideal candidates for waveguides.[75] Lieber’s group pioneered a work on CdS NW waveguides, and first reported that CdS NW could work as “active waveguides” when operated near the band-edge through the quantitative detection of optical intensity at different propagation distances, and the electronics was effectively combined with these active CdS NW waveguides to yield electro-optical modulations (EOMs), and efficient nanoscale lighting-emitting diode injection sources.[76] Later, Zou and co-workers quantitatively and systematically studied the guided band-edge luminescence using individual CdS NB by imaging the spatially localized photoluminescence intensity and simultaneously recording Figure 11. (a) Far-field PL image and (b) micro-PL spectrum of an excited superlattice CdS/ the PL spectra (energy and intensity) at dif- CdS:SnS microwire; (c) Schematic representation of the emission process of the 1D superlat2 ferent traveling distances.[77] They found that tice wire; (d, e) waveguiding mapping of band emission (501–506 nm) and valley region of both spectral position and the peak intensity multi-peak range (575–590 nm). Reproduced with permission.[78] Copyright 2010, American changed significantly by varying the excitation Chemical Society.



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an ideal hybrid structure consisting of the bottom-up synthesized CdS NW emitter (diameter of 80 nm, length of 5 μm, emission of 510 nm) and the top-down fabricated Si3N4 2D photonic-crystal freestanding slab waveguide (Figure 12a).[79] The 2D photonic-crystal structure was designed to overlap the CdS emission wavelength of 510 nm. The structure parameters of the photonic crystal, with a lattice constant of 185 nm and a hole diameter of 92.5 nm (inset of Figure 12b), was calculated to produce a bandgap of around 510 nm. The authors measured the injection and guidance of light from CdS NW into the photonic-crystal waveguide, and found that the light was rarely scattered in the middle of waveguide (Figure 12c). The PL spectrum in Figure 12d measured at the large hole (output) confirms the efficient coupling from the CdS NW to the waveguide. Furthermore, the electrically driven CdS NW delivered light in such photonic crystals. This hybrid NW/photonic-crystal waveguide not only revolutionized the field of solid-state lighting but also guided the future for nanophotonics and all-optical processing.[79,80] Further development in waveguide materials and structures can provide flexibility in connecting various nanophotonicselements and versatility of nanophotonics devices, which will eventually promote the development of optical information technology.

3.5. Field-Emitters

Figure 12. Optical injection from a CdS NW into the photonic-crystal waveguide: (a) Schematic of 2D photonic bandgap structure made in a Si3N4 slab with straight line-defect waveguide aligned with the CdS NW; (b) SEM image of CdS NW facing the straight waveguide terminated with a large hole used as an output scatter and magnified SEM images of CdS NW (inset, left) and photonic-crystal structure (inset, right). The inset scale bars, 500 nm; (c) PL superimposed on the SEM image of b. Scale bars, 5 μm; (d) PL spectra of the NW emission and the light scattered from the photonic-crystal waveguide output. Reproduced with permission.[79] Copyright 2008, Macmillan Publishers.

periodicity for an individual CdS/CdS:SnS2 superlattice wire. Several periodic green spots were separated by the dark regions, and each spacing between every two bright spots was about 7.3 μm; that is, equal to the length of CdS segment. The authors indicated that the CdS segments in the superlattices formed many optical microcavities in sequence that could confine and transport the photons, while SnS2 with a large refraction index worked as both reflection end-faces and emission centers. The periodic bright emissions arose from the interference of coherently scattered light waves on the end-faces of the microcativities and SnS2 emission (Figure 11b). As shown in Figure 11c, the superlattice wire exhibited a strong CdS band edge emission at 509 nm and multi-peak range of 525–650 nm. Images of the selected waveguiding (Figure 11d) show the allowed bands at 501–506 nm and forbidden bands at 575–590 nm. These emission profiles were produced by the combination of 1D photonic crystal and periodical exciton confinement.[78] Optically driven active CdS NW can be used to light up the photonic-crystal waveguides. Park and co-workers constructed

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Field-emission (FE) is based on the physical phenomenon of quantum tunneling, during which the electrons pass from an emitting material surface into a vacuum in the presence of a high electric field.[81–83] The electron emitters made of materials with a low work function and a high field-enhancement factor are expected to exhibit the excellent FE performances with low applied turn-on voltages and high current densities.[84] CdS has a relatively low work function of 4.2 eV, as compared with other many popular FE materials including C NTs (5.0 eV), ZnO (5.3 eV), ZnS (7.0 eV), placing it as one of the best FE candidates. We fabricated highly ordered CdS nanostructure arrays with different morphologies (inset of Figure 8a) in the similar growth conditions and systematically investigated the correlation between the morphologies, alignment and the FE performance.[30] The 1D CdS nanostructures of various types displayed notable differences in field-emission behaviors, as shown in Figure 13. The CdS emitters with higher aspect ratio and better alignment exhibited better FE performance with the lowest turn-on field of 12.2 V/μm, the lowest threshold field of 15.7 V/μm and the highest J at the same E value. Furthermore, we have fabricated single-crystalline CdS multipods, NBs, and nanotips and investigated their FE performances.[50,85,86] The CdS multipods were fabricated by thermal evaporation of CdS powders without using any catalysts. The length and tip diameters of the arms in CdS multipods were precisely adjusted by tuning the growth time. Due to the needle-shaped arms and high crystallinity, the long armed CdS multipods exhibited better FE properties with a field-enhancement factor of 2100 and a turn-on field of 7.2 V/μm compared with those of short- and medium- armed multipods.[85] The CdS NBs were several tens to hundreds of micrometers in length, and 200 nm



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Figure 13. Field-emission performances of morphology-dependent CdS nanostructure arrays. The color in J-E curves corresponds to the color in SEM images and geometric sketches of CdS nanostructure arrays. Reproduced with permission.[30] Copyright 2009, American Chemical Society.

in thickness. The NBs showed low turn-on of 3.7 V/μm and threshold voltages of 9.3 V/μm, and a high field enhancement factor of 1298, due to their larger aspect ratios.[86] The CdS nanotips were composed of branched structures with needleshaped tips. They possessed better FE characteristics with a relatively low turn-on field of 5.28 V/μm and the highest fieldenhancement factor of 4819 among all previously studied 1D CdS nanostructures.[50] The field emission from the surface of 1D nanostructures can be enhanced by the incidence of the light due to their intrinsic photoconductivity.[87–91] Joag and co-workers first investigated the photo-sensitive field emission behaviour of CdS nanocombs by exposing the cathode to a visible light of constant intensity.[92] Upon illumination, the turn-on field was decreased from 0.26 V/μm (without illumination) to 0.24 V/μm at a current density of 0.1 μA/cm2, the maximum current density was increased from 14.6 μA/cm2 (without illumination) to 26.9 μA/cm2 at a field of 0.65 V/μm, as shown in Figure 14a. Such light-induced effect on the field emission was reproducibly observed. The detailed photo-sensitive field emission mechanism is shown in Figure 14b, indicating that, upon continuous illumination of the cathode, more electrons were excited from the valence band into the conduction band, thus affecting the field emission and significantly increasing the current.[92] Very recently, we systematically discussed the factors affecting the FE performances, including the nanostructure morphology (tip geometry, alignment, density, diameter, length), phase structure, temperature, effects of light, gas, substrate, gap, composition (decorating with nanoparticles, doping, alloying), and the presence of hetero-/branched structures, and showed the ways of FE performance optimization. The detailed discussion can be found in chapter 23 of the recently published book.[2] 3.6. Logic Gates Logic gates are important building blocks in the integrated circuits. They are usually constructed of field-effect transistors 3026


Figure 14. (a) J-E curves of CdS nanocombs taken with and without light illumination; (b) Schematic of the photo-sensitive field emission mechanism. Reproduced with permission.[92] Copyright 2011, Royal Society of Chemistry.

(FETs) with electrically controlled “ON” and “OFF” switching functions.[93] High performance logic circuits have been fabricated on the basis of CdS nanostructures, and these individual nanodevices revealed advanced multifunctions. Ma and coworkers first demonstrated the construction of high-performance CdS logic gate (inverter), NOR and NAND gates based on two or three high-performance CdS NW metal-semiconductor FETs (MSFETs) with top Schottky gates employed.[94] They controlled the nanoFETs individually by local modulating the channel conductance of the single NWs, and then achieved the high-performance logic gates.[95,96] The CdS NW MSFETs showed excellent transistor parameters, such as a large on/off current ratio of 107, low threshold voltage of −0.4 V, and small subthreshold swing of 60 mV/dec. The inverter based on two MSFETs demonstrated a voltage gain of 83.[94] Wu and co-workers further improved the voltage gain of the CdS nanoinverter to 1000 through using high-κ dielectric HfO2 film as the top-gate oxide layer.[97] As shown in Figure 15a, two CdS NB metal-oxide-semiconductor FETs (MOSFETs) were used as the load and the driver, respectively, to construct the high-performance nanoinverter. The inverters exhibited many prominent characteristics, such as a large supply voltage range (from 50 mV to 10V), a high voltage gain of 1000 at the supply voltage of 10 V (Figure 15b), a low power consumption, and a good dynamic behavior with square wave


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Figure 15. (a) Schematic illustration and corresponding circuit diagram of an inverter made on a single CdS NW; (b) Voltage transfer characteristic (VTC) curves of the inverter at VDD = 10 V, demonstrating the high voltage gain of 1000. Reproduced with permission.[97] Copyright 2010, American Chemical Society.

input at frequencies up to 1 kHz, demonstrating the promising application prospect for future low power and high performance logic circuits.[97]

Figure 16. (a) IDS-VGS curves in three different double sweep ranges of a CdS NB-based floating nanodot gate memory (FNGM) device. The memory windows under ±1, ±3, ±5 V VGS double sweeps are about 1, 1.1 and 3.2 eV, respectively. The inset shows the schematic of CdS NB-based FNGM device; (b) Stressing characteristics of the FNGM were checked by applying a ±5 V erase/write voltage with 200 ms duration and a period of 2 s. Reproduced with permission.[99] Copyright 2010, Royal Society of Chemistry.

3.7. Memory Device Floating nanodot gate memory (FNGM) based on a metal-oxidesemiconductor (MOS) structure has drawn great attention due to its superior characteristics.[98] High-performance nonvolatile CdS NB-based FNGM was first reported by Wu and co-workers.[99] This structure consisted of a CdS field-effect transistor and Au nanodots embedded in high-k HfO2 top-gate dielectric, as shown in the inset of Figure 16a. A simple thermal evaporation method was employed to fabricate high-density uniformly distributed Au nanodots in between a 5 nm HfO2 tunneling layer and a 15 nm HfO2 control oxide layer. Figure 16a shows the IDS-VGS curves in three different double sweep ranges (VDS = 0.5 V), demonstrating that the memory characteristics became more and more obvious with an increase of gate sweep range, which originate from the tunneling of the charges between the CdS NB channel and the Au nanodots. Under a low operation voltage of 5 V, such FNGM device has a large memory window of 3.2 V, a long retention time of up to 105 s, and good stress endurance of more than 104 write/erase cycles

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(Figure 16b). Such merits make the CdS NB-based FNGM an attractive alternative to current non-volatile memory devices.[99] 3.8. Photodetectors With a wide direct bandgap of 2.4 eV, CdS has been considered as a prospective material for visible-light photodetectors.[100] There has been a number of recent reports on optoelectronic behaviors of CdS nanostructures, as shown in Table 1. For example, we assembled CdS NBs into nanoscale photodetector with Ohmic contacts (Figure 17a).[86] A high photocurrent of 30 μA was recorded at a 1.0 V bias when the NB was illuminated with a light of energy above the threshold excitation energy, such as 490 nm visible light. ntacts. CdS nanobelts into nanoscale ts on optoelectronic behaviors of CdS nanostructures, as shown in Table 1. low series reCompared with a dark current,



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www.MaterialsViews.com Table 1. Key parameters of CdS nanostructured photodetector devices. Photodetectors CdS NB

Light of detection

Bias (V)

Dark current or conductance

Photocurrent or conductance

Rise time

Decay time

Responsivity

Gain

Ref.

488 nm

0.5

26 fA

70 nA

137 μs