Indeed, the RDS/RAS measurements reveal information about molecular ordering of DNA bases induced by the density of steps on silicon surfaces. All.
J.
Phys. IV France 132 (2006) 69-72
0EDP Sciences, Les Ulis ~ 0 1 10.105 : l/jp4:2006132013
In situ reflectance anisotropy spectroscopy monitoring of wide bandgap biomolecules on vicinal silicon surfaces S.D.Silaghil, M. Friedrichl and D.R.T. Zahn1 Institut für Physik, TU Chemnitz, 09 107 Chemnitz, Germany
Abstract. DNA base molecules, adenine, thymine, guanine, and cytosine may be employed as charge transport molecules in biomolecular electronic devices. Their electronic properties compete with those of inorganic wide bandgap materials, e.g. GaN, with the absorption onset in the near ultra-violet (UV) range. A recent field effect transistor study based on a modified DNA base revealed that the Prototype bio-transistor gives rise to a better voltage gain compared to a carbon nanotube one (CNT) [I]. Reflectance Anisotropy Spectroscopy (RDSIRAS) measures the difference in reflection for normally incident light linearly polarized along two orthogonal directions in the sample surface as a function of photon energy. in situ RDSIRAS is employed under ultra-high vacuum (UHV) conditions for the first time for the characterization of DNA base molecules on vicinal hydrogen passivated Si(] 11) surfaces. Such vicinal substrates consisting of steps and terraces can provide a versatile template for molecular ordering. Indeed, the RDS/RAS measurements reveal information about molecular ordering of DNA bases induced by the density of steps on silicon surfaces. All four molecules behave differently on the vicinal substrates. The orientation of the transition dipole moments of the molecules with respect to the substrate directions can be evaluated from the RDS/RAS spectra. For adenine and thymine the transition dipole moments align mainly perpendicular to the step edge direction while for guanine and cytosine they align parallel to this direction, however, only in very thin layers. The RDSIRAS signal of the guanine and cytosine layers with thicknesses above 20 nm saturates due to the loss of ordering.
1. INTRODUCTION The usage of DNA base molecules as charge transport molecules in biomolecular electronic and optoelectronic devices is promising but still a challenge. The capability to fabricate well-ordered structures with characteristic dimensions of a few nanometers is a key prerequisite for future applications in nanoelectronics or as functional materials on a nanometer scale. Silicon can be patterned in many ways and it is thus possible to use it as a versatile template. Flat Si(1 11) has a 3-fold symmetry inducing the growth of three equivalent superstructure domains by symmetry but vicinal S i ( l l 1 ) surfaces can limit overlayer growth to a single domain. Moreover, vicinal surfaces should favour nucleation along the step edges, thus being potential substrates in controlling the ordering of molecules in so-called molecular nano-wires. Here, in situ reflectance anisotropy spectroscopy (RDSIRAS) is employed under UHV conditions for monitoring the ordering of DNA base molecules on vicinal hydrogen passivated S i ( l 1 I).
2. EXPERIMENTAL Vicinal p-type (B-doped) Si (I 1 1) wafers with resistivity in the range of 1-30 R c m were used as substrates for the DNA base films. Double-side polished substrates off cut oriented by 3" and 6", respectively, towards the /Ti21 direction were supplied by Silchem. Prior to biomolecular deposition, the substrates were wet-chemically hydrogen terminated 131. The surface reconstruction ( 1 X 1) was checked by low energy electron diffraction (LEED) showing a double splitting of the diffraction points in the [I121 direction typical for the formation of steps and terraces. The source materials of high-purity DNA base powders p~irchasedfrom Across Organics were evaporated under UHV conditions (base pressure 1 0-"a) from Knudseii cells. Molecules of thymine and cytosine were evaporated at temperatures of
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365 and 410 K with corresponding evaporation rates of 0.8 and 0.3 nm/min, respectively, while adenine arid guanine molecules were evaporated at temperatures of 400 and 5 10 K with evaporation rates of 1.5 and 2 nm/min, respectively. The molecular ordering of DNA base molecules on vicinal H:Si(I 1 1 ) surfaces was monitored in situ during the growth by reflectance anisotropy spectroscopy (RDS/RAs) in the energy range of 1.5-5.5 eV. RDS/RAS measures the difference in reflection for light normally incident linearly polarized along two orthogonal directions in the sample surface. In the case of a vicinal Si(l1 I) surface the RDS/RAS signal can be expressed as follows:
3. RESULTS The anisotropy of cubic materials, e.g. silicon, arises at the surface due to a broken symmetry as in the case of vicinal Si(] 11) substrates where the surface anisotropy is induced by the formation of steps and terraces. Such anisotropy is often referred to as surface induced optical anisotropy (SIOA) [4, 51. The RDS/RAS signal of vicinal Si(l11) surfaces is similar to the response of the Si(l10) surface (see fig. 1). Figure 2 depicts the RDS/RAS monitoring of DNA bases on H:Si( 111)-6"surfaces. All four molecules behave optically different on the vicinal surface. Small changes in anisotropy are observed in the case of guanine and cytosine deposition in comparison with the large signals arising from adenine and thymine layers. When increasing the guanine coverage the RDS/RAS signal reaches a saturation level for thicknesses above 30nm. Changes can be observed around the critical points of silicon which overlap with the absorption of guanine. The highest occupied molecular orbital (HOMO) -+lowest unoccupied molecular orbital (LUMO) transition of guanine at 4.3 1 eV is very close to the E2 (4.25 eV) critical point (CP) of silicon [6]. Figure 3 sumrnarizes the orientation of the HOMO -+LUMO transition dipole moments of DNA bases as well as the corresponding experimental and theoretical energy positions. The lineshape of the signal evolves in a derivative-like lineshape with increasing guanine coverage above 14 nm. For coverage above 20nm the signal Starts to decrease and finally saturates (fig. 2a). This is likely to be related to the loss of ordering. The RDS/RAS measurements of guanine deposition on "flat" H:Si(lll)-0.35" revealed features related to thickness artefacts [6]. The weak anisotropy of guanine films on vicinal substrates seems to be caused by vicinality which induces ordering of the molecules up to thickness of -20nm along [110]. Contrary to guanine, adenine exhibits very strong anisotropy signals. By increasing the adenine coverage the RDS/RAS signal evolves in a derivative-like lineshape in the absorption range of adenine
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(HOMO -+LUMO transition -4.47eV) [6]. Following the surface dielectric anisotropy model [7] it is found that the dominant absorption takes place mainly along [ i i 2 ] direction suggesting furthermore strong ordering of adenine molecules with the transition dipole moment parallel to this direction. Weaker signals with similar lineshape were also observed on H:Si(111)-0.35" [6]. The RDS/RAS monitoring of cytosine (HOMO --+LUMO transition -4.46eV) deposition on H:Si(l 11)-6" shows that the lineshape of the signal is strongly thickness-dependent as in the case of guanine (fig. 2c). At a certain cytosine coverage a new feature around 4.90eV appears which is close to the second electronic transition in cytosine at
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4.95eV [6]. For very thin cytosine coverages, the molecules seem to preferentially align with the fiisi transition dipole moment along [ I f 01. For coverages larger than l Onm the signal starts to decrease arid finally saturates for thicknesses above 20nm (fig. 2c). Strong RDS/RAS signals were also observed in the case of thymine layers on H:Si(111)-6" as reproduced in fig. 2d. The lineshape of RDS/RAS signal is very much like that of the dielectric function E of thymine [6]. The sign of RDS/RAS signal suggests high absorption along [ I 121, hence a preferential alignment of the transition dipole moment (HOMO + LUMO transition-4.44 eV) with respect to this direction. Smaller anisotropies were also observed on H:Si(111)-0.35" [6]. 4. SUMMARY The RDS/RAS technique was employed for monitoring the growth of DNA bases on vicinal H:Si(ll]). The measurements revealed information about the molecular ordering of DNA bases induced by the density of steps on vicinal surfaces. For a more precise determination of the molecular orientatio,, RDS/RAS measurements in an extended energy range towards ultra-violet should be performed.
Acknowledgments The financial support by SMWK, DFG GRK 82911 "ACCUMOL" is acknowledged.
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