Synthesis of CdSXSe1−X Nanoribbons with Uniform and Controllable

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Synthesis of CdSXSe1-X Nanoribbons with Uniform and Controllable Compositions via Sulfurization: Optical and Electronic Properties Studies Guohua Li,† Yang Jiang,*,† Yi Wang,† Chun Wang,† Yangping Sheng,† Jiansheng Jie,‡ Juan Antnio Zapien,§ Wenjun Zhang,*,§ and Shuit-Tong Lee§ School of Materials Science and Engineering, Hefei UniVersity of Technology, Hefei 230009, China, Department of Applied Physics, Hefei UniVersity of Technology, Hefei 230009, China, and Center of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong, SAR, China ReceiVed: May 18, 2009; ReVised Manuscript ReceiVed: July 26, 2009

Ternary CdSXSe1-X single-crystal nanoribbons (NRs) with uniform and controllable compositions (0 < X < 1) were synthesized for the first time via sulfurizing the CdSe nanoribbons. The product was characterized by means of X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Analysis results revealed that the CdSXSe1-X nanoribbons had a wurtzite structure and grew along the [0001] direction. Photoluminescence measurements showed the CdSXSe1-X NRs had tunable and sharp near-band gap emissions and lasing action from 542 to 668 nm. Metal oxide semiconductor field-effect transistor devices based on single CdS0.25Se0.75 nanoribbons showed a pronounced gating effect, a threshold voltage of 4.9 V, a transconductance of 95 nS, and an on-off ratio of 105. Electron mobility and carrier concentration of the CdS0.25Se0.75 NR were estimated to be 14.8 cm2/(V s) and 3.9 × 1016 cm-3, respectively. 1. Introduction Semiconductor nanowires (NWs) and nanoribbons (NRs) have shown great potential as building blocks for new, nanoscaled electronic and optoelectronic devices.1 Thus far, devices such as light-emitting diodes, photodetectors, lasers, field-effect transistors (FETs), solar cells, and memory devices have been demonstrated using one-dimensional (1D) semiconductor nanostructures.2–5 To apply the nanostructures in nanoelectronics and nanophotonics, we found it is essential to synthesize nanostructures with controllable and tunable physical properties. Recently, it has been revealed that optical and electrical properties of ternary semiconductor nanocrystals and films can be tuned by varying the composition of ternary compounds.6a,11a However, there are relatively few reports on 1D ternary semiconductor nanostructures with controlled composition and physical properties. One-dimensional ternary compounds of group II-VI materials have been synthesized by several research groups. ZnXCd1-XS single-crystal NRs with variable composition have been synthesized by combining laser ablation of CdS with thermal evaporation of ZnS powder.11a Alloyed CdSXSe1-X NRs have been produced from CdS and CdSe powders with lasing properties at low (77 K)12a and room12b temperatures covering the full composition range (0 < X < 1). Alloyed CdSXSe1-X NWs have been synthesized by a pulsed laser deposition process in a hot wall-type chamber.13 Wavelength-tunable photoluminescence and band gap modulation have been demonstrated in alloyed ZnXCd1-XS and CdSXSe1-X. However, the synthesis of ternary nanostructures with uniform structure and controlled composition is still an important challenging task, which is * To whom correspondence should be addressed. E-mail: apjiang@ hfut.edu.cn (Y. Jiang), [email protected] (W. J. Zhang). † School of Materials Science and Engineering. ‡ Department of Applied Physics. § City University of Hong Kong.

difficult to achieve because of the inherent growth mechanism and inevitable growth fluctuation. For instance, a common drawback in the above-mentioned synthesis methods is that the products of different compositions of variable X are invariably codeposited on the same substrate, and the composition X changes with the distance of the substrate from the evaporation sources. The products obtained are thus inevitably a mixture of various compositions; such products would limit the application of the as-synthesized 1D ternary semiconductor nanostructures in practical nanodevices and other fields where nanostructures of uniform and controlled physical properties are required. In this study, we present a new sulfurization method, which enables for the first time the synthesis of 1D ternary CdSXSe1-X single-crystal NRs with uniform and controllable compositions. The approach consists of two successive steps involving (i) synthesizing single-crystal CdSe NRs by using thermal evaporation and (ii) sulfurizing the CdSe NRs by annealing in a H2S-Ar atmosphere to achieve the ternary-alloyed CdSXSe1-X NRs. Although the shape and size of the CdSe NRs can be controlled as well, in this paper, the focus is on the control of the element composition. In the second step, Se atoms in CdSe are partially substituted by S atoms. While the single-crystal nature of the NRs is maintained, the content of sulfur in the NRs could be controlled accurately by varying the annealing temperature and duration. The CdSXSe1-X NRs obtained by this method possess a uniform composition (stable X) over the substrate. Significantly, photoluminescence (PL) measurements demonstrate the ternary CdSXSe1-X NRs to exhibit tunable near-band gap emission and tunable lasing from 542 to 668 nm. Moreover, electronic and transport properties of the CdSXSe1-X NRs were studied by examining the field-effect transistors (FETs) fabricated from single CdSXSe1-X NRs.

10.1021/jp9046402 CCC: $40.75  2009 American Chemical Society Published on Web 09/04/2009

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2. Experimental Section CdSXSe1-X NRs with highly uniform composition were fabricated using a two-step process as follows. First, CdSe NRs were synthesized via the vapor-liquid-solid (VLS) plus vapor-solid (VS) approach by thermal evaporation of CdSe. The experiment setup consisted of a horizontal tube furnace with a quartz tube mounted inside and a gas flow control system. The CdSe powder (Aldrich, 99.99%) source was placed in the middle of the quartz tube, while the silicon substrates coated with 30 nm gold film were placed in downstream about 10 cm from the source powders. After the furnace system was evacuated to a pressure of 1.0 × 10-3 Pa by a mechanical pump and turbo molecular pump, an argon flow at 50 sccm was introduced, with the pressure controlled by continual pumping. The temperature at the center of the tube was increased to 850 °C in 40 min and maintained at this temperature for 2 h. In the second step, the as-synthesized CdSe NRs were annealed in a constant atmosphere of 10% H2S and 90% Ar of 200 Torr at different temperatures of 700, 750, and 800 °C. However, it was revealed that the sulfurization process was too fast at the annealing temperature of 800 °C; as a result, the composition of the CdSXSe1-X NRs was difficult to control. In contrast, the sulfurization process at 700 °C was very slow, which made the experiments less efficient. To balance efficiency and controllability of the reaction, we selected to anneal at 750 °C, and the composition X of the CdSXSe1-X NRs was controlled by adjusting the annealing time. The morphologies and microstructures of the as-synthesized products were characterized by field-emission scanning electron microscopy (FE-SEM, SIRION 200), transmission electron microscopy (TEM, Philips CM20 operated at 200 kV), highresolution transmission electron microscopy (HRTEM, JEOL2010 operated at 200 kV), and X-ray diffraction (XRD, D/maxrB). Power-dependent photoluminescence (PL) measurements were conducted at room temperature using the fourth harmonic of an Nb:YAG (yttrium aluminum garnet laser; wavelength, 266 nm) with a 6 ns pulse width as the excitation source. The excitation laser was focused onto the sample with a diameter of ∼200 µm. The PL signal was detected with a UV optical fiber leading to a 0.5 m spectrograph using a 1200 groove mm-1 grating (Acton Research, Spectra Pro 500i) and an intensified charge-coupled device (CCD) detector. Measurements were performed using normal incident excitation and a 10° detection angle. For the fabrication of FET devices, CdSXSe1-X NRs were dispersed uniformly onto a SiO2 (300 nm)/Si (p) substrate at a desired density. The SiO2/Si substrates were then spin-coated with photoresist, baked, exposed to photolithography for the definition of electrode arrays, and developed. The indium (80 nm) and Au (10 nm) electrodes were deposited on the two ends of the NR by e-beam evaporation, and the remaining photoresist lifted off in acetone. The back gate electrode was fabricated by e-beam evaporation of gold on the p-Si side of the substrate. Then a fast annealing at 250 °C was carried out in an N2 atmosphere for 4 min to improve the contact between the electrodes and NRs. An additional annealing in vacuum (3.0 × 10-6 Torr) at 400 °C for 30 min was adopted to further improve the performance of the single-nanoribbon FETs. The parameters of the FET devices were measured on a Keithley 4200 semiconductor characterization system. 3. Results and Discussion Sulfurization was conducted by annealing the as-synthesized CdSe NRs in the H2S/Ar atmosphere at a constant temperature

Figure 1. Normalized XRD patterns of CdSXSe1-X (0 < X < 1) nanoribbons obtained by annealing in H2S in an Ar atmosphere for different durations of (a) 180, (b) 120, (c) 90, and (d) 60 min. The x values are calculated from the XRD data by using Vegard’s law.

but for different duration ranging from 30 to 240 min. Figure 1 depicts the X-ray diffraction (XRD) patterns of four CdSXSe1-X (0 < X < 1) samples annealed for different durations of 180, 120, 90, and 60 min. The diffraction patterns of CdS (JCPDS card 41-1049) and CdSe (card 77-0046) are also presented as reference. It shows that all samples are in the hexagonal wurtzite structure. As the annealing duration increases, the diffraction peaks of the samples shift gradually toward a higher value of 2θ, indicating that the lattice constants of the NRs decrease and approach those of CdS. For CdSXSe1-X ternary compounds, a well-defined relationship, i.e., Vegard’s law,14 has been established between the composition value X and lattice parameters (c-axis).

Cx ) CCdSe + (CCdS - CCdSe)X

(1)

where CCdse, CCdSe,, and Cx are the C-axis lattice constants of hexagonal CdSe, CdS, and CdSXSe1-X, respectively. Therefore, X values of the NRs can be determined using the lattice parameters deduced from the corresponding XRD data. For samples shown in panels a-d of Figure 1, the X values are calculated to be 0.56, 0.34, 0.25, and 0.18, respectively, which are in good agreement with the energy dispersive X-ray spectrum (EDS) results shown below. An important point to note is that all NRs collected from different sections on a same substrate show the same XRD patterns (Figure S2 of the Supporting Information). This same XRD data means uniform chemical composition of NRs was achieved by sulfurization on the same substrate. These indicate that postannealing of CdSe NRs in a sulfur-containing atmosphere is a straightforward and reliable approach to synthesize alloyed CdSXSe1-X NRs with controllable and desirable chemical compositions. Table 1 summarizes the composition of the CdSXSe1-X NRs synthesized by annealing for different durations. It can be seen that X values can be controlled to vary from 0.12 to 0.82 by increasing the annealing time from 30 to 240 min.

Synthesis of CdSxSe1-X Nanoribbons

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TABLE 1: Lattice Constants, PL Peaks, and Band Gaps of CdSXSe1-X Nanoribbons Produced with Different Annealing Durations PL band annealing lattice time (min) constant Cx (Å) composition X max (nm) gap (eV) 30 60 90 120 150 180 210 240

6.975 6.958 6.937 6.911 6.885 6.847 6.786 6.771

0.12 0.18 0.25 0.34 0.43 0.56 0.77 0.82

668 658 647 627 610 594 559 542

1.86 1.88 1.92 1.98 2.03 2.09 2.22 2.28

Panels a and b of Figure 2 show the scanning electron microscopic (SEM) morphologies of NRs obtained after annealing for 120 and 90 min, respectively. The NRs have smooth surfaces, thicknesses of 50-90 nm, widths of 0.5-2 µm, and lengths up to several tens of micrometers. The elemental compositions of these NRs were probed by energy dispersive X-ray spectrum (EDS). The results show that all of the samples contain Cd, Se, and S, and the atomic ratios of (Se + S)/Cd are all very close to 1. The X values estimated from the EDS spectra are ∼0.34 and 0.25 for the NRs in panels a and b of Figure 2, respectively, which are in good agreement with those calculated on the basis of XRD measurements. Panels c and d of Figure 2 show the high-resolution transmission electron microscopic (HRTEM) images of CdS0.34Se0.66 and CdS0.25Se0.75 NRs, respectively. The insets are their corresponding selected area electron diffraction (SAED) patterns. It can be seen that the NRs grow along the [0001] direction, and both NRs are singlecrystals in a hexagonal structure. This means the single NR is one type of single crystal compound rather than a mixture from two types of compounds. On the basis of the experimental conditions and above results, the sulfurization process of CdSe NRs in a H2S-Ar gas mixture can be roughly considered to be a three-step reaction process (shown in Figure 3) associated with the following reactions

Figure 2. FE-SEM images and EDS spectra (insets) of CdSXSe1-X nanoribbons with compositions of (a) X ∼ 0.34 and (b) X ∼ 0.25. HRTEM images and selective area electron diffraction (SAED) patterns (insets) of two single CdSXSe1-X nanoribbons with compositions of (c) X ∼ 0.34 and (d) X ∼ 0.25, showing their high-quality single-crystalline nature and growth directions.

H2S f H2 + S

(2)

CdSe + XS f CdSXSe1-X + XSe

(3)

Se + H2 f H2Se

(4)

First, H2S molecules are decomposed into H2 molecules and S atoms (reaction 2). S atoms then diffuse into the NRs and substitute Se atoms (reaction 3). At the annealing temperature, the S atoms can penetrate the entire NR thickness easily and uniformly due to the small thickness of NRs. The released Se atoms react with H2 molecules to form gaseous H2Se (reaction 4). More Se atoms could be replaced by S atoms as the sulfurization time increases. A similar mechanism has been proposed for the synthesis of CuIn(SYSe1-Y)2 thin films.15 According to reactions 2-4, sulfurization is a complicated dynamic equilibrium process in which the X value and reaction speed depend on many factors, including temperature, vapor pressures of H2S and H2Se, diffusion of S atoms into NRs, and substitution of Se atoms by S atoms. Composition of the products can be controlled through varying the sulfurization times. However, CdSe NRs can be hardly transferred completely into CdS NRs, and the maximum X value achieved under our experimental conditions is 0.93 after annealing for 24 h. Room-temperature PL properties, photonic properties, and the laser model from the single of CdS, CdSe, and CdSSe NWs/ NRs have been intensively studied.16–18 Herein, we present our PL results based on the single CdSXSe1-X NR. Room-temperature PL spectra of the CdSXSe1-X NRs, excited using a Nd: YAG laser (266 nm) and synthesized with annealing times of 240, 210, 180, 150, 120, 90, 60, and 30 min, are shown in Figure 4 (curves a-h, respectively). Normalized spectra of all samples show a sharp band gap emission that shifts from ∼542 to ∼668 nm (2.28 to 1.86 eV), which indicates a narrowing of the band gap of CdSXSe1-X NRs as the S content decreases (X f 0). The maximum PL emission peaks and corresponding calculated band gap energies (Eg) are summarized in Table 1, and similar results for ternary CdSXSe1-X films have been reported.10 Yet, the emission position is different from the observed defectrelated emission band in CdSSe nanowires18c and two emission

Figure 3. Schematic shows the three reactions in the sulfurization process of CdSXSe1-X NRs.

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Figure 4. Normalized PL spectra of CdSXSe1-X nanoribbons excited with a Nd:YAG laser (266 nm). Curves ash are for the CdSXSe1-X NRs synthesized by annealing for 240, 210, 180, 150, 120, 90, 60, and 30 min, respectively. The data above the PL peaks show the X values in the corresponding sample.

peaks in the Si-CdSSe core/shell nanowires.19 Meanwhile, in order to ascertain the uniform composition of CdSXSe1-X on the same substrate, we investigated XRD and PL properties of samples from different sections on the same silicon substrate. The silicon substrate was partitioned into four sections labeled a, b, c, and d as shown in Figure S1 of the Supporting Information. XRD and room-temperature PL measurements were conducted on the four samples. PL spectra demonstrate the same shape and peak locations as shown in Figure S3 of the Supporting Information. The same XRD patterns and PL spectra from the four sections on the same substrate show the products are uniform in chemical composition. That uniform composition was achieved in the product is the key distinction of the present synthesis from the previous ones11–13 in which products of nonuniform compositions were typically produced in one synthesis. We have also studied the PL spectra of the CdSXSe1-X NRs with increasing excitation power densities; the resulting data for the CdS0.12Se0.88 and CdS0.82Se0.18 NRs are shown in Figure 5. The PL peak narrows and shows superlinear intensity dependence on the excitation power density with an increase in the excitation power, indicating the appearance of lasing action. Lasing action in self-assembled nanowires and nanoribbons results from the Fabry-Perot optical cavity defined by their well-faceted ends and high optical gain at high optical pumping densities.1 At high excitation power densities, the CdS0.12Se0.88 and CdS0.82Se0.18 NRs show lasing modes centered at ∼668 and ∼542 nm, respectively, demonstrating that the CdSXSe1-X NRs act as an optical cavity with stimulated amplified emission. Lasing phenomena were observed in CdYS1-YSe, ZnXCd1-XS, ZnS, and CdS NRs,4a,11a which are characterized by (i) a broadband emission at low excitation power densities below the lasing threshold,(ii) the presence of sharp resonant modes in the PL spectra above the lasing threshold, indicating the high lasing quality of the optical cavity defined by the NRs, and (iii) a superlinear dependence of the emission intensity, which determines the onset of amplified stimulated emission. In our experiments, the lasing threshold for the CdS0.12Se0.88 and CdS0.82Se0.18 NRs are ∼32 and ∼40 kW/ cm2, respectively. It was noticed in our experiments that the NRs with higher S contents showed higher threshold values.

Figure 5. (a) Room-temperature PL spectra showing band gap emissions from CdSXSe1-X nanoribbons with compositions (a) X ∼ 0.12 and (b) X ∼ 0.82 excited by a Nd:YAG laser (266 nm) at different power densities. Insets: Integrated emission intensity as a function of the optical pumping power density.

The lasing threshold value is a function of material quality and optical-cavity quality (such as low reflectivity at the cavity ends).4d For the nanoribbons in Figure 5, it is likely that changes in optical cavity quality arising from different morphologies and sizes (as per Figure 2) are responsible for the changes in the lasing threshold because no apparent difference in the material quality of the products was detected, and they appear single crystal with hexagonal structure. Further PL investigations indicate that the lasing in these CdSXSe1-X NRs can be continuously tuned by composition tuning covering the spectral range from 542 to 668 nm. Because the excitation spot size was 200 µm in diameter, many NRs with a distribution of sizes and orientations with respect to the measuring direction were simultaneously excited. Therefore, it is reasonable that individual resonant modes cannot be resolved in Figure 5b, where emissions resulting from the leaky modes with no preferential direction are also collected. Nevertheless, the sharp features marked by the dashed line in Figure 5a in the PL spectra recorded at high excitation powers correspond to the resonant cavity modes in the CdS0.12Se0.88 NRs. Such resonant emission is preferentially detected for those NRs with their axis aligned in the direction of the optical fiber as a consequence of the high directionality of the lasing emission in NRs. The mode spacing is 1.7 nm, and the corresponding longitudinal mode of a Fabry-Perot optical cavity can be estimated from the following equation4d

∆λ ≈ λ2 /2nL

(5)

Synthesis of CdSxSe1-X Nanoribbons

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17187 increases (decreases) as Vg becomes more positive (negative). Thus, the current saturation and Vg dependence behavior are characteristic of an n-channel FET.21,22 The Ids versus Vg relation (Vds ) 0.8 V) on an exponential scale is shown in Figure 6 b, from which a threshold voltage (VT) of 4.9 V and an on-off ratio of 105 are obtained for this device. The VT (∼4.9 V) of the device based on CdS0.25Se0.75 NR is lower than those based on the CdSe NR (VT ∼ 20.9 V) and CdS NR (VT ∼ 18.3 V) in our previous reports,5e,d and the on-off ratio (105) is an order of magnitude higher than those of the CdSe NR and CdS NR. Electron mobility and carrier concentration (ne) can be estimated from the following equations

Figure 6. (a) Ids-Vds curves at Vg from 0 to 26 V at steps of 2 V. Left inset: FE-SEM image of a single CdSXSe1-X NR FET. Right inset: Schematic of a single CdSXSe1-X NR-based FET. (b) Log Ids-Vg curves (Vds ) 0.8 V).

where L is the length of the laser cavity, n is the refractive index of CdS0.12Se88 (abouit 2.5), λ is the resonant wavelength (about 668 nm), and ∆λ (1.7 nm in the present case) is the spacing between adjacent resonant modes.20 Accordingly, L is estimated to be 52 µm, which is consistent with the length of CdSXSe1-X NRs obtained in our experiments. Compared to the previous report,11 the PL data from the CdSXSe1-X NRs, obtained on our new developed synthesis method, show obvious improvement in lasing characteristics. Electrical and transport properties of the ternary CdSXSe1-X NRs were evaluated by studying the performance of FETs fabricated from single NRs. The insets in Figure 6a show the SEM image and schematic of a typical CdS0.25Se0.75 NR device. The source and drain electrodes separated by 2 µm were formed on a NR of 1.4 um in width and 90 nm in thickness. Typical gate-dependent source-drain current (Ids) versus voltage (Vds) curves measured at various gate voltages (Vg) from 0 to +26 V (in steps of 2 V) on the CdS0.25Se0.75 NR are shown in Figure 6a. The measurements were performed in ambient air. Significantly, the device shows a pronounced gating effect; that is, when Vg increases (or decreases), the conductance of the NR increases (or decreases) correspondingly. Such gate-dependent Ids-Vds characteristics reveal the n-type conductivity of CdSXSe1-X. Similar to other II-VI and oxide semiconductor nanostructures, vacancy-induced donors are considered to induce such transfer characteristics.5g Examination of individual Ids versus Vds curves shows that the current begins to saturate at positive but not negative values of Vds and that conductance

µn ) gmC0-1Vds-1Z/L

(6)

ne ) C0VT /eT

(7)

where C0 is the gate capacitance, T is the thickness of the NR, Z/L is the ratio of channel width to channel length (1.4/2), and gm () ∂Ids/∂Vg) is the transconductance of the FET.23 Assuming a parallel plate capacitor model, the capacitance per unit area is given by C0 ) εε0/h, where ε is the dielectric constant, and h is the thickness of the SiO2 dielectric layer; thus, C0 is about 11.5 nF/cm2 for our device configuration. The gm value is estimated to be ∼95 nS at Vds ) 0.8 V. We obtained µn ≈ 14.8 cm2/(V s) and ne ≈ 3.9 × 1016cm-3 on the basis of the above equations and Ids-Vd curves in Figure 6 b. The electron mobility (µn) of the CdS0.25Se0.75 NR is slightly larger than those of the pure CdSe NR [9.6 cm2/(V s)]5e and CdS NR [1.7 cm2/(V s)].5d Although the mobility obtained here is still much lower than the Hall electron mobility in CdSe [∼900 cm2/(V s)]28 and CdS [∼340 cm2/(V s)] bulks23 and In-doped CdS NRs [∼330 cm2/ (V s)],29 it is comparable to those of CdS0.25Se0.75 [20 cm2/(V s)], CdSe, and CdS film devices.24–27 There are only a few reports on the electronic and transport properties of ternary compound NRs based on single NR FET studies thus far. We believe the tunable band gaps together with reasonably large carrier mobilities of CdSXSe1-X NRs enable their potential applications in new electronic and optoelectronic nanodevices. 4. Conclusions A new sulfurization method was developed to synthesize CdSXSe1-X NRs with variable composition (0 < X < 1) using single-crystal CdSe NRs prepared by thermal evaporation as the starting material. This process enables accurate control of the composition of the CdSXSe1-X NRs. All CdSXSe1-X NRs are single crystals growing along the [0001] direction. The incorporation of S into CdSe NRs was considered to proceed with the diffusion of S atoms into the NRs and substitution of Se atoms by S atoms at the annealing temperature. This effective route to achieve alloy nanostructures may be extended to synthesizing other 1D ternary alloys. Photoluminescence measurements showed the CdSXSe1-X NRs exhibit wavelengthtunable sharp near-band gap emission and lasing action shifting continuously from 542 to 668 nm. Such NRs should find significant applications in wavelength-tunable nano-optoelectronic devices in the visible region. FET devices based on a single CdS0.25Se0.75 NR showed a pronounced gating effect, a threshold voltage of 4.9 V, a transconductance of 95 nS, and an on-off ratio of 105. Electron mobility and carrier concentration of the CdS0.25Se0.75 NR are estimated to be 14.8 cm2/(V s) and 3.9 × 1016 cm-3, respectively. The tunable band gaps together with reasonably large carrier mobilities of CdSXSe1-X

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NRs enable their potential applications in new electronic and optoelectronic nanodevices. Acknowledgment. The work was financially supported by the National High Technology Research and Development Program of China (No.2007AA03Z301), the Natural Science Foundations of China (No. 20771032, No.60806028) and Anhui Province (070414200), and the National Basic Research Program of China (No.2007CB9-36001). S.T. Lee would like to acknowledge the supports from Research Grants Council of Hong Kong SAR (NSFC-RGC: N_CityU125/05), China, US Army International Technology Center-Pacific and the National High-tech R&D Program of China (863 Program) (Grant 2006AA03Z302). J.S. Jie would like to thank the financial support from program for new century excellent talents in university of the Chinese Ministry of Education (No. NCEF08-0764). Supporting Information Available: Figures of a typical sample on a silicon substrate, XRD of CdS0.18Se0.82 nanoribbons, and photoluminescence of CdS0.18Se0.82 nanoribbons. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Law, M.; Goldberger, J.; Yang, P. Annu. ReV. Mater. Res. 2004, 34, 83. (b) Fan, H. J.; Werner, P.; Zacharias, M. Small 2006, 2, 700. (2) (a) Zhong, Z.; Qian, F.; Wang, D.; Lieber, C. M. Nano Lett. 2003, 3, 343. (b) Kim, H.; Cho, Y.; Lee, H.; Kim, S.; Ryu, S. R.; Kim, D. Y.; Kang, T. W.; Chung, K. S. Nano Lett. 2004, 4, 1059. (c) Hayden, O.; Greytak, A. B.; Bell, D. C. AdV. Mater. 2005, 17, 701. (3) (a) Jiang, Y.; Zhang, W. J.; Jie, J. S.; Meng, X. M.; Fan, X.; Lee, S. T. AdV. Funct. Mater. 2007, 17, 1795. (b) Wang, J.; Gudiksen, M. S.; Duan, X.; Cui, Y.; Lieber, C. M. Science 2001, 293, 1455. (4) (a) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature 2007, 449, 885. (b) Ma, R.-M.; Dai, L.; Huo, H.-B.; Xu, W. J.; Qin, G. G. Nano Lett. 2007, 7, 3300. (c) Law, M.; Jonhson, D. J.; Goldberger, J.; Saykally, R. J.; Yang, P. Science 2004, 4, 197. (d) Johnson, J. C.; Yan, H. Q.; Yang, P.; Saykally, R. J. J. Phys. Chem. B 2003, 107, 8816. (5) (a) Gunawan, O.; Sekaric, L.; Majumdar, A.; Rooks, M.; Appenzeller, J.; Sleight, J. W.; Guha, S.; Haensch, W. Nano Lett 2008, 8, 1566. (b) Xiang, J.; Lu, W.; Hu, Y.; Lieber, C. M. Nature 2006, 441, 25. (c) Hu, Y.; Xiang, J.; Liang, G.; Yan, H.; Lieber, C. M. Nano Lett 2008, 8, 925. (d) Jie, J. S.; Zhang, W. J.; Jiang, Y.; Lee, S. T. Appl. Phys. Lett. 2006, 89, 223117. (e) Jie, J. S.; Zhang, W. J.; Jiang, Y.; Lee, S. T. Appl. Phys. Lett. 2006, 89, 133118. (f) Jiang, X.; Xiong, Q.; Nam, S.; Qian, F.; Li, Y.; Lieber, C. M. Nano Lett. 2007, 7, 3214. (g) Arnold, M. S.; Avouris, P.; Pan, Z.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659. (6) (a) Bailey, R. E.; Nie, S. J. Am. Chem. Soc. 2003, 125, 7100. (b) Zhong, X.; Feng, Y.; Knoll, W.; Han, M. J. Am. Chem. Soc. 2003, 125, 13559. (7) (a) Murali, K. R.; Venkatachalama, K. Chalcogenide Lett. 2008, 5, 181. (b) Ezema, F. I.; Osuji, R. U. Chalcogenide Lett. 2007, 4, 69. (8) Petrov, D. V.; Santos, B. S.; Pereira, G. A.; Dongega, C. D. M. J. Phys. Chem. B 2002, 106, 5325. (9) Pan, A.; Liu, R.; Wang, F.; Xie, S.; Zou, B.; Zacharias, M.; Wang, Z. L. J. Phys. Chem. B 2006, 110, 22313.

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