NANO LETTERS
Self-Assembled Monolayers of CdSe Nanocrystals on Doped GaAs Substrates
2002 Vol. 2, No. 8 911-914
Eike Marx,† David S. Ginger,† Karsten Walzer,‡ Kurt Stokbro,‡ and Neil C. Greenham*,† CaVendish Laboratory, Madingley Road, Cambridge, CB3 0HE, United Kingdom, and Mikroelektronik Centret (MIC), Technical UniVersity of Denmark, Bldg. 345 east, DK-2800 Lyngby, Denmark Received June 25, 2002
ABSTRACT This letter reports the self-assembly and analysis of CdSe nanocrystal monolayers on both p- and n-doped GaAs substrates. The selfassembly was performed using a 1,6-hexanedithiol self-assembled monolayer (SAM) to link CdSe nanocrystals to GaAs substrates. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), optical ellipsometry, photoluminescence (PL), and scanning tunneling microscopy (STM) measurements were used to confirm stable, disordered, densely packed CdSe nanocrystal monolayers separated from the substrate by a SAM of 1,6-hexanedithiol.
There is increasing demand to reduce the size of the active elements in electronic circuits to the nanometer scale in order to continue current rates of progress in circuit performance. Both organic molecules and inorganic nanocrystals have been investigated as suitable active elements. Devices have typically been fabricated using planar metal electrodes to contact a monolayer of organic molecules,1,2 or using nanometer-sized gaps between metal electrodes to contact small numbers of molecules or nanocrystals.3,4 Semiconductor electrodes may in the future provide an attractive alternative to metals, since nanometer-sized gaps may be fabricated by selective etching of semiconductor heterostructures in wafer-scale processes that are compatible with traditional semiconductor processing methods.5 These structures might show interesting electrical properties, perhaps similar to the negative differential resistance that is observed in resonant tunneling diodes where a quantum-confined system is placed between two doped semiconductor “leads”. Characterization of nanocrystals and molecules attached to semiconductor surfaces is therefore an important area of research, and here we study the assembly of CdSe nanocrystals on GaAs surfaces using alkanedithiols as linkers. The self-assembly of molecules such as alkanethiols on metal surfaces is well-established and has been used to attach CdSe nanocrystals, for example, to gold surfaces,6 and to measure their electrical properties.3,4 Various authors have * Corresponding author. E-mail
[email protected]. † Cavendish Laboratory. ‡ Technical University of Denmark. 10.1021/nl025669d CCC: $22.00 Published on Web 07/11/2002
© 2002 American Chemical Society
Figure 1. Schematic of the GaAs/1,6-hexanedithiol/nanocrystal structure to be assembled.
also demonstrated assembly of monolayers of alkanethiols and other molecules on GaAs surfaces.7,8 Furthermore, Reifenberger and co-workers9-12 have described and characterized the self-assembly of gold nanoclusters on p- and n-doped GaAs using thiol linking molecules. In this letter, we aim to describe the assembly and thorough characterization of semiconductor nanocrystals attached to semiconducting substrates. Figure 1 shows the target structure, where an alkanedithiol layer is used to attach CdSe nanocrystals to a GaAs substrate. CdSe nanocrystals were synthesized by the tri-n-octylphosphine-oxide (TOPO) method of Murray13 et al. as modified by Katari et al.14 Their diameter was determined by analysis of the absorption spectra. Polished 〈100〉 GaAs wafers were used, either p-doped (Be, 1 × 1018 cm-3) or n-doped (Si, 1 × 1018 cm-3). 1,6-Hexanedithiol 96%, 2-propanol 99.5% HPLC grade, and toluene 99.8% HPLC grade were purchased from Aldrich Chemicals. Aqueous ammonia solution (35%) and hydrochloric acid (HCl, 32%) were purchased from BDH. All chemicals were used without further purification. The GaAs wafers were cleaned in an
Figure 2. Infrared spectra of the CH2 stretching mode region for a self-assembled monolayer of 1,6-hexanedithiol on n-doped GaAs (top) and for a clean n-doped GaAs wafer (bottom). Thin lines are measured data and thick lines are smoothed over approximately 5 cm-1.
Figure 3. PL of 4 nm diameter CdSe nanocrystals (bottom) with a peak at 601 nm, n-doped GaAs (middle) with a peak at 855 nm, and assembled CdSe nanocrystals on n-doped GaAs (top) with two peaks, one at 601 nm and the other at 855 nm.
ultrasonic bath for 10 minutes with acetone and 10 minutes with 2-propanol and dried in a stream of nitrogen. Surface oxide was then removed by a 60 s etch in concentrated (32%) HCl, followed by a rinse with deionized water and a stream of nitrogen to dry the samples. Surface modification of the GaAs wafers was performed in a solution containing 88% 2-propanol, 12% aqueous ammonia solution, and 8 × 10-4% 1,6-hexanedithiol (volume/ volume) for 4 h in darkness and at room temperature. The solution was purged with argon prior to the modification. The surface modification was followed by several thorough rinses with 2-propanol. The self-assembly of the CdSe nanocrystals was performed in a toluene solution containing 0.016 wt % CdSe nanocrystals for 12 h in darkness and at room temperature. Two different sized CdSe nanocrystals were used. The large and small CdSe nanocrystals had first absorption peaks at 588 and 517 nm respectively, corresponding to diameters of ∼4 nm and ∼2.4 nm.15 After assembly the samples were thoroughly rinsed with toluene and dried with a stream of nitrogen. For comparison between chemically bonded and physically adsorbed CdSe nanocrystals a second surface modification was performed without using alkanedithiols. After the HCl etch, the doped GaAs samples were immersed in the nanocrystal solution for 36 h and then rinsed as before. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and optical ellipsometry were used to investigate the effects of the surface treatment with 1,6hexanedithiol. ATR-FTIR measurements were performed using a Nicolet 5SXB FTIR in combination with an Specac ATR accessory, and gave a resolution of 2 cm-1. Variableangle spectroscopic ellipsometry was performed using a J. A. Woollam M-2000 ellipsometer at angles of 45°, 50°, 55°, 60°, and 65°. Figure 2 shows the IR spectrum for the treated surface. CH2 stretching modes at 2855 and 2929 cm-1 confirm the presence of 1,6-hexanedithiol on the surface. The shift of
the peaks in comparison to the modes observed in liquid 1,6-hexanedithiol is due to the packing of the 1,6-hexanedithiol chains on the GaAs surface,16 consistent with the formation of an organized layer. Similar results have previously been reported for the formation of alkanethiol monolayers on nondoped GaAs.8 Ellipsometry was performed on freshly etched GaAs wafers, and the change in polarization state of the reflected beam was modeled using an iterative technique to find the optical constants of the GaAs, using the literature values as a starting point. Measurements were then made on samples with 1,6-hexanedithiol attached. The results were modeled at a wavelength of 633 nm, taking the refractive index of the 1,6-hexanedithiol layer to be 1.5120,17 and the absorption coefficient to be zero. From the fitting process, the thickness of the layer was found to be 4-5 Å. The layer thickness was then fixed in the model, and the optical constants of the 1,6-hexanedithiol layer were fitted over the wavelength range from 450 to 800 nm. The layer thickness is smaller than the length of the molecule (∼8 Å), indicating either partial coverage of the surface or assembly of the molecules at an angle to the surface normal. Similar results have been seen in other studies where chemically bonded molecules are found to lie at an angle to the substrate.16,18-21 Our results are thus consistent with assembly of a monolayer of 1,6hexanedithiol on doped GaAs at an angle of 50-60° to the surface normal. The deposition of CdSe nanocrystals on the 1,6-hexanedithiol monolayer was investigated with photoluminescence (PL) measurements, ellipsometry, and scanning tunneling microscopy (STM). Photoluminescence was excited with an argon ion laser (488 nm, 500 mW) and measured using an Ocean Optics S2000 spectrometer. The PL measurements were performed under vacuum (5 × 10-5 mbar) with an integration time of 150 ms and an average over 250 scans. Figure 3 shows the PL of 4 nm diameter CdSe nanocrystals assembled on n-doped GaAs, compared with the PL of the
912
Nano Lett., Vol. 2, No. 8, 2002
Figure 4. PL of (A) 2.4 nm and (B) 4 nm diameter CdSe nanocrystals, (C) p-doped GaAs, (D) 2.4 nm diameter nanocrystals on p-doped GaAs, (E) 4 nm diameter nanocrystals on p-doped GaAs, (F) n-doped GaAs, (G) 2.4 nm diameter nanocrystals on n-doped GaAs, and (H) 4 nm diameter nanocrystals on n-doped GaAs.
substrate and of the nanocrystals in solution. The PL of the CdSe nanocrystals attached to the GaAs is identical to the spectrum measured in solution, consistent with the lack of any strong electronic interaction between the nanocrystals and the substrate. Similar behavior is seen with p-doped substrates and with smaller nanocrystals, as illustrated in Figure 4. The control sample with no alkanedithiols shows no CdSe luminescence, which demonstrates that the nanocrystals do not absorb to the GaAs surface in the absence of the 1,6-hexanedithiol layer. Although the PL measurements demonstrate the presence of nanocrystals on the surface, they cannot quantify the thickness and the coverage of the CdSe nanocrystal layer. Ellipsometry was therefore used to determine the optical constants and the thickness of the CdSe nanocrystal layer. The substrate and alkanedithiols were modeled as before, and an additional layer was added to represent the nanocrystals. Optical constants for nanocrystals are not well known, and therefore had to be determined independently. First, a Kramers-Kronig consistent oscillator model (comprising 5 Lorentzian oscillators) was used to reproduce the shape of the measured absorption spectrum in solution. To reproduce the absolute values of n and k for the solid film while maintaining both the shape of the absorption spectrum and Kramers-Kronig consistency, the optical constants were combined using the Bruggeman effective-medium approximation with a void (n ) 1, k ) 0) and a constant refractive index layer (n ) 6, k ) 0). The model was fitted to the experimental data between 450 and 800 nm and determined the fraction of the three components in the layer as well as the thickness of the layer. The optical constants determined for the layers are shown in Figure 5, and the layer thicknesses were found to be of 4.29 ( 0.04 nm and 2.53 ( 0.02 nm for large and small nanocrystals, respectively. To assess the coverage of the GaAs substrate with CdSe nanocrystals, STM was performed using a Rasterscope 4000. Nano Lett., Vol. 2, No. 8, 2002
Figure 5. Real (n, dashed line) and imaginary (k, solid line) parts of the complex refractive index of CdSe nanocrystal layers with nanocrystal diameters of 2.4 nm (circles) and 4 nm (squares), as determined by fitting of ellipsometry and UV-vis data.
Figure 6. STM image of (A) a clean p-doped GaAs substrate; and (B) 4 nm diameter CdSe nanocrystals assembled on p-doped GaAs with a 1,6-hexanedithiol monolayer. The tunnel current was 1 nA for A and 0.18 nA for B with a sample bias of 2.5V.
The topography of the samples was measured immediately after assembly, under ultrahigh vacuum (UHV) with a base pressure below 2 × 10-10 mbar, and at room temperature. The tungsten tips were transferred to UHV immediately after etching them for 1 min with 5% hydrofluoric acid. Part A in Figure 6 illustrates the blank p-doped GaAs substrate, and part B shows the coverage of 4 nm CdSe nanocrystals on the substrate. The images shown are typical of images measured in several different areas of the substrate. Part A shows a peak-to-valley roughness of less than 0.5 nm, which was unchanged by 1,6-hexanedithiol assembly (data not shown). Part B shows features with heights of up to 1.3 nm and typical lateral sizes of 4-5 nm. Together with the ellipsometry results, this image is consistent with a highcoverage disordered nanocrystal monolayer on the surface, given the estimated STM tip diameter of 20 nm. 913
We have described the assembly of CdSe nanocrystals on GaAs substrates using a monolayer of 1,6-hexanedithiol to provide chemical attachment of the particles to the semiconductor surface. The films have been characterized with ATR-FTIR, photoluminescence, optical ellipsometry, and STM, and the results are consistent with the formation of stable, disordered monolayers of CdSe nanocrystals with a high packing density. This assembly technique may have applications for the production of molecular electronic devices on semiconductor substrates. Acknowledgment. This work was supported by the European Commission IST program (IST-1999-10323, “SANEME”), and by the Engineering and Physical Sciences Research Council, UK. We thank the Semiconductor Physics Group, Cavendish Laboratory for supplying the GaAs wafers. References (1) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550-1552. (2) Collier, C. P.; Mattersteig, G.; Wong, E. W.; Luo, Y.; Beverly, K.; Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000, 289, 1172-1175. (3) Klein, D. L.; McEuen, P. L.; Katari, J. E. B.; Roth, R.; Alivisatos, A. P. Appl. Phys. Lett. 1996, 68, 2574-2576. (4) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699-701. (5) Sazio, P. J. A.; Berg, J.; See, P.; Ford, C. J. B.; Lundgren, P.; Greenham, N. C.; Ginger, D. S.; Bengtsson, S.; Chin, S. N. Mater. Res. Soc. Symp. Proc. 2001, 679, B2.3.1.
914
(6) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221-5230. (7) Sheen, C. W.; Shi, J. X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514-1515. (8) Baum, T.; Ye, S.; Uosaki, K. Langmuir 1999, 15, 8577-8579. (9) Lee, T.; Liu, J.; Janes, D. B.; Kolagunta, V. R.; Dicke, J.; Andres, R. P.; Lauterbach, J.; Melloch, M. R.; McInturff, D.; Woodall, J. M.; Reifenberger, R. Appl. Phys. Lett. 1999, 74, 2869-2871. (10) Lee, T.; Chen, N. P.; Liu, J.; Andres, R. P.; Janes, D. B.; Chen, E. H.; Melloch, M. R.; Woodall, J. M.; Reifenberger, R. Appl. Phys. Lett. 2000, 76, 212-214. (11) Janes, D. B.; Lee, T.; Liu, J.; Batistuta, M.; Chen, N. P.; Walsh, B. L.; Andres, R. P.; Chen, E. H.; Melloch, M. R.; Woodall, J. M.; Reifenberger, R. J. Electron. Mater. 2000, 29, 565-569. (12) Janes, D. B.; Batistuta, M.; Datta, S.; Melloch, M. R.; Andres, R. P.; Liu, J.; Chen, N. P.; Lee, T.; Reifenberger, R.; Chen, E. H.; Woodall, J. M. Superlattices Microstruct. 2000, 27, 555-563. (13) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715. (14) Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 4109-4117. (15) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343-5344. (16) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-65. (17) Aldrich Handbook of Fine Chemicals; Sigma Aldrich: Dorset, U.K., 1999. (18) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45-52. (19) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (20) Ulman, A. AdV. Mater. 1993, 5, 55-57. (21) Ulman, A. Chem. ReV. 1996, 96, 1533-1554.
NL025669D
Nano Lett., Vol. 2, No. 8, 2002
