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Investigation of Transversal Conductance in Semiconductor CdTe Nanowires with and without a Coaxial Silica Shell Xiaorong Liang,†,§ Susheng Tan,† Zhiyong Tang,‡ and Nicholas A. Kotov*,‡ Chemistry Department, Oklahoma State University, Stillwater, Oklahoma 74078, and Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48198 Received October 12, 2003. In Final Form: December 11, 2003 Semiconductor CdTe nanowires were coated with a coaxial layer of silica in aqueous solutions by the hydrolysis of tetraethyl orthosilicate. The structure and morphology of the nanowires before and after coating were studied by UV-vis spectroscopy, atomic force microscopy, and transmission electron microscopy. Interestingly, silica deposition on the nanowire ends was retarded. For sufficiently thin layers, this effect makes possible the preparation of coaxial insulated nanowires with electrical access to the semiconductor core. Insulating properties of the silica layer were tested by applying voltage to a conductive AFM tip placed directly on the sidewall surface of the nanowires. These experiments demonstrated that the silica layers had strong insulating properties for coatings as thin as 10 nm. Similar measurements done with naked nanowires showed that transversal conductance took place through the middle band-gap states.
Introduction Semiconductor nanowires (NWs) are one of most basic elements of future electronic devices. They are often made and processed in the form of colloids, the starting point for many electronic and photonic applications. NWs can accommodate on their body functional circuit elements, for instance, nanoparticles, as well as current control elements, such as gate contacts. While NW computers belong to a distant reality, more practical applications of NWs include sensors exhibiting exceptional sensitivity.1-5 They alter their conductivity and/or photophysics upon adsorption of different species to their surfaces. The influence of external conditions and surrounding molecules on NW properties is beneficial for sensing but must be tightly regulated when NWs are employed for other purposes. For instance, field effects and temperaturedependent surface adsorption/desorption equilibrium will produce strong cross talk and operational errors. Their influence in nanocircuits will be far stronger than in conventional ones. Insulation of NWs with a layer of inert material should prevent/reduce the strong noise in nanocircuits. Such a layer can protect the surface from adsorption of unwanted species, stop unnecessary charge injection, and partially screen the external fields. For sensor applications, a thin insulation layer can also be used to improve the environmental stability of the devices.6-8 One of the materials most suitable for NW protection is silica. It has high voltage * To whom correspondence should be addressed. E-mail: kotov@ umich.edu. † Oklahoma State University. ‡ University of Michigan. § Current address: Department of Chemistry, Purdue University, West Lafayette, IN, 47907. (1) Hu, J. T.; Odom, W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435-445. (2) Gudiksen, M. S.; Lauhon, U. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617-620. (3) Dai, H. Surf. Sci. 2002, 500, 218-241. (4) Wu, Y.; Yan, H.; Huang, M.; Messer, B.; Song, J. H.; Yang, P. Chem.s Eur. J. 2002, 8, 1260-1268. (5) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353-389. (6) Wang, D.; Salgueirino-Maceira, V.; Liz-Marzan, L. M.; Caruso, F. Adv. Mater. 2002, 14, 908-912. (7) Correa-Duarte, M. A.; Kobayashi, Y.; Caruso, R. A.; Liz-Marzan, L. M. J. Nanosci. Nanotechnol. 2001, 1, 95-99.
breakdown potential and dielectric constant,9 considerable mechanical strength, and exceptional resilience to environmental factors. Moreover, silicon oxide processes are common for the modern electronics industry and can potentially be transferred to the new generation of devices. However, SiO2 is intrinsically hydrophilic, and its selective deposition encounters difficulties when realized in a vacuum or in hydrophobic solvents typical for nanowire colloids. Nevertheless, Ag, SiC, Si, and other NWs have been coated with a shell of silica to meet different requirements.10-12 All of these NWs except Ag are processed in nonaqueous solvents or solid-state materials via vapor-phase or high-temperature routes. Very recently, subsequent deposition of silica and metal in porous alumina templates was explored to prepare insulating Au nanowires.13 Besides silica, some polymers can also be considered for making the insulating layer. As such, CdSe/ poly(vinyl acetate)14 and Au/polystyrene15 core-shell onedimensional structures with small aspect ratios were produced. The actual insulating capabilities of the organic or inorganic shell strongly depend on its quality and remain unknown for both types of coatings. Results and Discussion 1. Preparation and Characterization of NWs with a Coaxial Layer of Silica. A simple method of synthesis of semiconductor NWs with high aspect ratios, from 1:300 to 1:800, by self-assembly from CdTe nanoparticles (NPs) was introduced in our previous publication.16 The resulting CdTe NW dispersions are made in aqueous media and therefore conveniently lend themselves to the production of insulated NWs, whose preparation and characterization (8) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 1999, 103, 6770-6773. (9) Dimaria, D. J. Solid-State Electron. 1997, 41, 957-965. (10) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208-211. (11) Meng, G. W.; Zhang, L. D.; Mo, C. M.; Zhang, S. Y.; Qin, Y.; Feng, S. P.; Li, H. J. J. Mater. Res. 1998, 13, 2533. (12) Yin, Y.; Lu, Y.; Sun, Y.; Xia, Y. Nano Lett. 2002, 2, 427-430. (13) Kovtyukhova, N. I.; Mallouk, T. E.; Mayer, T. S. Adv. Mater. 2003, 15, 780-785. (14) Xie, Y.; Qiao, Z. P.; Chen, M.; Liu, X. M.; Qian, Y. T. Adv. Mater. 1999, 11, 1512-1515. (15) Obare, S. O.; Jana, N. R.; Murphy, C. J. Nano Lett. 2001, 1, 601-603. (16) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237-240.
10.1021/la035908s CCC: $27.50 © 2004 American Chemical Society Published on Web 01/22/2004
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are described in this communication. In addition to the deposition of a uniform silica shell around the NW, the quality of the insulating layer was verified by in situ current measurements with an atomic force microscopy (AFM) probe. The preparation and formation mechanism of CdTe NWs by spontaneous restructuring of NPs has been described elsewhere.16 Briefly, 5-10 mL of methanol was added into 5 mL of 8.6 × 10-6 M NPs.17 The mixture was centrifuged for 10 min at 5800 rpm. The precipitate was then redissolved in 5 mL of pH 10.0 deionized water. After that, the reaction medium was placed in an oven for 3 days at 55 °C. The solution darkened, which indicated the formation of NWs. CdTe NWs obtained by this method were uniform in diameter but varied in length from 50 nm to several micrometers depending on the stabilizer, being on average 1 µm in length. The silica coating process followed the recipe developed for metal and semiconductor NPs by the groups of P. Mulvaney and L. Liz-Marzan.6-8 Initially, the original stabilizer of NWs (thioglycolic acid) was exchanged with a primer, (3-mercaptopropyl)trimethoxysilane (MPS). Two milliliters of CdTe NW solution was added into 5 mL of ethanol, followed by adding 50 µL of MPS and 50 µL of tetraethoxysilane (TEOS). Two drops of ammonia hydroxide were added to expedite the reaction. After several hours, the solution became opaque. The mixture was centrifuged and redissolved in 8 mL of ethanol. The deposition of silica on the CdTe NWs was found to be frustrated without the primer. The molecular mechanism of the silica polymerization on the colloidal particles is investigated in detail in the works of Prof. Liz-Marzan and Mulvaney.8,19 The preparation of the silica shell was achieved via basecatalyzed hydrolysis of TEOS, also known as the Sto¨ber reaction.18 It produces small silica clusters, which further hydrolyze and condense on the MPS-coated CdTe NW. Ammonia was used to speed up the coating process. In the case of a thick silica shell (>50 nm), the product formed a precipitate due to an increase of NW mass. After centrifugation, the precipitate could be redispersed in ethanol, forming transparent stable solutions. Silicacoated NWs were placed on an As-doped silica wafer by using a spin coater. Then the sample was transferred for 20 min into the oven at 100 °C for water evaporation before the conductive AFM measurements were carried out. Figure 1 shows the UV-vis absorption and photoluminescence spectra of thioglycolic acid (TGA) stabilized CdTe NPs, NWs, and silica-coated CdTe NWs. Compared with the original CdTe NPs, the NWs showed a 30 nm red shift in both UV-vis absorption and photoluminescence spectra due to the relaxation of quantum confinement conditions in the longitudinal direction in NWs. After silica coating, the absorption characteristics are dominated by scattering rather than absorption, which is reflected by the flattened spectrum. The photoluminescence peak can be clearly seen in emission and remains at the same wavelength as the luminescence of the original NW dispersions with significantly reduced luminescence intensity due to chemical effects of SiO2 on the CdTe surface and light scattering. (17) The molar concentration of NPs was calculated assuming full conversion of Cd2+, the density of the CdTe in the NPs being equal to that in the bulk material, and the diameter of CdTe spheres being equal to 5 nm. (18) Stoeber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62-69. (19) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740-3748.
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Figure 1. UV-vis absorption (A) and photoluminescence (B) spectra of TGA-stabilized CdTe NPs, NWs, and silica-coated NWs.
Typical transmission electron microscopy (TEM) images of the NWs after the silication process are shown in Figure 2B-D, which can be compared to the analogous image of uncoated NWs (Figure 2A). The silica layer coats the sides of the NW surface with a continuous layer, whose thickness may vary substantially along the axis (Figure 2C) especially in the initial stages of the deposition. Importantly, the silica layer on the ends of the NW is consistently thinner in every sample (Figure 2B,C), which is likely related to the absorption preference of the MPS primer. Nonuniform adsorption of thiols on the NW surface has been demonstrated in the study of centipede-like SiO2coated NWs from our group.20 One can optimize the TEOS concentration and silica deposition time to make a colloid in which the ends remain open. Examples of such particles are presented in Figure 2C. When the silica layer becomes thicker, the overgrowth of the coating on the sidewall closes the cap on the ends (Figure 2B). Little or no deposition on the NW ends opens the possibility of electrical access to the semiconductor core. If necessary, a monolayer of thiolic stabilizer as well as fortuitous SiO2 deposits can be removed from them by etching. This step will be beneficial for making ohmic contact to the NWs. The physical integrity of the coaxial composite even for long wires with a thin coating (Figure 2D) is indicative of the continuity of the CdTe core underneath the silica sheath. No evidence of cross-linking of different composite rods can be seen in TEM for a low concentration of TEOS, which is contrary to silica coatings of nanoparticles leading to occasional encapsulation of multiple metal or semiconductor cores.19 (20) Wang, Y.; Tang, Z.; Liang, X.; Liz-Marza´n, L. M.; Kotov, N. A. Nano Lett., ASAP Article 10.1021/nl0349505; web release date: 01/ 07/2004.
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Figure 2. TEM images of CdTe NWs before (A) and after (B-D) silica coating. Panels A and B are survey images, and panels C and D are the images of single NWs coated with thick (100-200 nm, panel C) and thin (10-30 nm, panel D) layers of silica. The scale in the panel C insert is 150 nm.
Both sensing and electronic applications of NWs require variations in the degree of insulation. Among other means, the control over the shell thickness can be done by changing the molar ratio of TEOS to CdTe NWs. When the molar concentration of TEOS changes from 5 mM to 0.4 M for a given amount of NWs, the thickness of the silica shell increases from 2 to 65 nm. The thicker SiO2 layer also significantly decreases the roughness of the coating. The nonuniformity of the silica coating (Figure 2D) indicates the heterogeneity of the semiconductor surface and is related to variability in the SiO2 nucleation rate over the NW surface.20 2. Conductivity Measurements in Coated and Naked NWs. To substantiate the claim that coaxial silicacoated NWs are indeed sufficiently insulated, direct conductance measurements should be performed. One can place a NW between two planar lithographically patterned electrodes as was done by Kovtyukhova et al.13 for 90280 nm diameter Au wires coated with SiO2. However, reproducible positioning of the colloidal particle between two electrodes is a nontrivial procedure. Additionally, the small diameter of the semiconductor NWs used here (3-5 nm) and the large spacing between the planar electrodes will lead to high resistivity of the system and much higher operational voltages than for noble metal NWs. Polarization and secondary chemical effects may distort the results, which will be difficult to interpret. To obviate this problem, the conductance measurements here were done by conductive AFM (C-AFM). Previous C-AFM studies on objects other than NWs demonstrated that it could be a powerful tool yielding direct information on surface conductivity, two-dimensional dopant profiling, patterning, and electronic states of nanoscale objects.21-23 Compared with (21) Ara, M.; Graaf, H.; Tada, H. Appl. Phys. Lett. 2002, 80, 25652567. (22) Kobayashi, K.; Yamada, H.; Matsushige, K. Appl. Phys. Lett. 2002, 81, 2629-2631. (23) Oh, J.; Nemanich, R. J. J. Appl. Phys. 2002, 92, 3326-3331.
scanning tunneling microscopy (STM), C-AFM separates acquisition of the sample topography from the measurements of the tunneling current. The C-AFM probe is in a mode of full contact with the substrate, which standardizes the spacing between the tip and the surface upon the set point of contact force, while in STM the tunneling gap is dependent on current conditions. Such decoupling of imaging from electrical probing makes the comparison of C-AFM curves obtained from different samples straightforward. The transverse conductivity with and without the silica shell was measured by applying a small voltage to a conductive AFM tip positioned directly on the NW surface. All the C-AFM i-V curves were obtained in several areas of the NW surface placing a tip on at least five different NWs; the most typical curves are presented below. AFM images of the NWs before and after silica coating were also obtained in the process of C-AFM measurements (Figure 3). The diameter obtained from AFM height profiles, that is, 6, 28, and 138 nm for naked, thinly coated, and thickly coated NWs, respectively, correlates well with the average diameters determined from TEM images (Figures 2A and 4), which is 5.5 ( 1 nm, 29 ( 4 nm, and 140 ( 5 nm for the same samples. This translates into silica shell thicknesses of 10 ( 2 nm and 66 ( 2.5 nm for the NWs examined by C-AFM (Figure 3B,C). The i-V curves for the As-doped silicon wafers that served as supports for the NWs show high currents for small voltages. Two plateaus for voltages beyond (0.2 V are due to the safety limit of the detection system (Figure 5). The i-V curve for the naked NW is noticeably asymmetric around 0 V: the current gradient is much greater for small negative potentials than for small positive ones. This asymmetry reflects the disparity of transport characteristic of electrons and holes through the waferNW-AFM tip system. The tunneling current through the naked NWs decreased 2 times compared with that of the Si wafer, due
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Figure 3. AFM images of a CdTe NW before (A) and after coating with 10 nm (B) and 66 nm (C) silica layers. All images are 600 nm × 600 nm.
Figure 4. Survey TEM images of CdTe NWs after coating with 10 nm (A) and 66 nm (B) silica layers.
Figure 5. C-AFM i-V curves for an As-doped silicon wafer (1), a naked CdTe NW (2), and both silica-coated NW samples (3). The bias in C-AFM is applied to the substrate while the probe is grounded.
to the organic layer of TGA on their surfaces. For CdTe NWs, coated with 66 or 10 nm thick layers of silica, no tunneling current was detected passing through with bias voltage reaching as high as (3 V (Figure 5), which correlates well with the data obtained by Olbrich et al.28 This observation confirmed the expectations that the SiO2 coating was strongly insulating. C-AFM measurements for NWs with a SiO2 thickness of less than 10 nm were not attempted because of the high roughness of the shell (Figure 2D) and the potentially patchy nature of the coatings. The data obtained here and in the literature21 indicate that a layer of silica within 2-5 nm should be
Figure 6. Differential i-V curves of the CdTe NW and the Si wafer.
sufficient to prevent unwanted charge transport at 0-5 V biases, which should be most typical for nanoscale devices. For the screening of field effects, a silica layer with a thickness of more than 10 nm, probably in the range of 50 nm and above, will be needed.24 The C-AFM i-V curves can also be used to determine the energy characteristics of the valence and conduction bands of the object under the C-AFM tip. Note that the definition of bias polarity in C-AFM is relative to the substrate, so the positive bias means that electrons flow from tips to substrate. The tunneling current can pass across NWs, when the negative voltage applied to the substrate is sufficient to put electrons on the conduction band and/or when the positive voltage is high enough to withdraw electrons from the valence band. For this analysis, it is more convenient to present the C-AFM i-V curves in the differential form (Figure 6). The differential i-V curve of the Si substrate shows a peak at +0.1 V and a similar peak at -0.1 V. As expected, the As-doped Si is highly conductive and has a band gap of 0.2 eV. The C-AFM peaks of Si are fairly symmetric in respect to the 0 V mark, demonstrating reasonably good alignment of the Fermi levels in the substrate and in the tip. The differential i-V spectrum of naked CdTe NWs placed on a doped Si wafer is substantially more complex than for insulated ones. There are two very strong peaks at 0.6 and -0.2 V and a group of relatively weak peaks between -1.0 and -0.5 V. The difference between the strongest peaks at 0.6 and -0.2 V is 0.8 V, which should be compared with the band gap of the NWs determined from their luminescence spectra (Figure 1), that is, 1.9 V. (24) Nagumo, T.; Hiramoto, T. Jpn. J. Appl. Phys., Part 1 2003, 42, 1988-1992.
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This discrepancy indicates that the actual transport from the AFM tip to the nanowire and then to the substrate is likely to occur through intra-band-gap states. At the same time, the difference between the most negative edge of the group of small peaks at -1.0 V and the strong most positive peak at 0.6 V gives a difference of 1.6 V, which correlates very well with the expected band gap of CdTe NW. This makes the small peak at -1.0 V and the peak at +0.6 V the edges of the band gap. Both states are fringed with a variety of other (surface) states visible as nonGaussian tails. The peak at -0.2 V represents the intra-band-gap states, which sustain the charge transport through the nanowire. Lower energy of these states facilitates the tunneling of the charge carriers. The existence of “conduction channels” is predicted for metal NWs in parts of the zones where bands cross the Fermi level.25 “Metallization”, that is, increase of free electron density, of the semiconductor NW and the appearance of additional populated states can originate from the direct contact with the AFM tip and related formation of highly localized interfacial dipoles and charge doping.26 In this system, one should also take into account the possibility of chemical doping. The presence of oxygen can significantly reduce the energy of the conductive states at selected points on the NW surface. Since C-AMF measurements were done under the ambient
conditions, the number of such states should actually multiply under the bias voltage due to progressive electrocorrosion, which can explain the relative strength of the -0.2 V peak. At the present moment, one should allow the possibility of all these processes considering the genesis of the mid-band-gap conduction peak. In conclusion, one of the most important areas of further research on this (and other) preparation procedures of coaxial insulated NWs should be improvement of the smoothness of the coating. Sufficiently uniform coatings will allow later stage processing of the nanowires and their successful organization in device elements. This task becomes especially obvious when comparing any example of sol-gel-derived SiO2 coating reported in the literature so far and the coaxial wires produce oxidation of silicon NWs.27 Excellent control of the silica layer on the Si substrate and a large amount of empirical data make oxidative coatings very uniform. This approach is limited only to Si NWs, while the sol-gel method can be used with virtually any material. Retarded silica deposition in the ends of the NWs makes potentially possible chemical processing of the NWs to effect electrical connectivity between them, retaining the insulating layer on the sidewalls. It is also important for understanding chemical reactivity of different surfaces of nanocolloids.
(25) Zhao, J.; Buia, C.; Han, J.; Lu, J. P. Nanotechnology 2003, 14, 501-504. (26) Landman, U.; Barnett, R. N.; Scherbakov, A. G.; Avouris, P. Phys. Rev. Lett. 2000, 85 (9), 1958-1961. (27) Ishii, K.; Suzuki, E.; Sekigawa, T. J. Vac. Sci. Technol., B 1997, 15 (3), 543-547. Teo, B. K.; Li, C. P.; Sun, X. H.; Wong, N. B.; Lee, S. T. Inorg. Chem. 2003, 42 (21), 6723-6728. Wu, C.; Qin, W.; Qin, G.; Zhao, D.; Zhang, J.; Xu, W.; Lin, H. Chem. Phys. Lett. 2003, 378 (3,4), 368-373. (28) Olbrich, A.; Ebersberer, B.; Boit, C. Appl. Phys. Lett. 1998, 73 (21), 3114-3116.
Acknowledgment. The authors are thankful to Professor Luis Liz-Marzan (University of Vigo, Spain) for the experimental guidance in the course of this project. The authors also thank the referees for stimulating comments. N.A.K. thanks NSF-CAREER, NSF-Biophotonics, NIH-NASA, AFOSR, and OCAST for financial support of this project. LA035908S
