Influence of Morphology on the Optical Properties of Self-GrownNanowire ArraysLiqiang Li,† Zhufeng Liu,† Ming Li,† Lan Hong,† Hui Shen,† Chaolun Liang,‡ Hong Huang,‡ Dan Jiang,‡
and Shan Ren*,†
†State Key Laboratory of Optoelectronic Materials and Technologies, The Key Laboratory of Low-carbon Chemistry & EnergyConservation of Guangdong Province, The Center for Nanotechnology Research, School of Physics Science and Engineering and‡Instrumental Analysis & Research Center, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, P. R. China
*S Supporting Information
ABSTRACT: Nanostructured materials such as nanowire arrays often absorblight more strongly than thin films, so they can be used to improve theperformance of solar cells. In this study, a series of self-grown Cu2S nanowirearrays with different diameters, lengths, and morphologies is prepared by solid−gas reaction between Cu foil and a mixture of H2S and O2. The structure of thearrays is characterized by XRD, TEM, XPS, and Raman. Their light absorptionperformance is investigated systematically by diffuse reflectance and photo-luminescence spectroscopy. The nanowire arrays are single-crystal Cu2Ssemiconductors, and their band gap can be adjusted by changing themorphology, diameter, and length of nanowires in the arrays. The light-absorption ability is enhanced from 70% for a planar Cu2S film to 93.5% for aCu2S nanowire arrays and is less sensitive to both the wavelength and incidentangle of light because of the morphology and distribution of the nanowires. Thelight absorption is high (about 92−95%) over a wide range of wavelengths (240−670 nm) and only decreases by 3% as theincident angle increases from 10 to 40°. This research shows the potential of Cu2S nanowire arrays for use in solar energyapplications.
1. INTRODUCTION
To improve the conversion efficiency of solar cells, variousnanostructures such as nanofilms,1−4 nanoparticles,5 nanowirearrays,6−12 nanotubes,13,14 and nanorod arrays14 have beenexplored extensively in recent years. One-dimensional nano-structures have received considerable attention due to theirdistinctive structure and properties that include enhanced lighttrapping, reduced light reflection, improved band gap tuning,facile strain relaxation, and increased structure defect tolerancecompared with those of bulk materials.15 One-dimensionalnanostructures also use previously disregarded low-costmaterials and processing options and can be applied inphotoconductors16 and field-emission devices.8
Various 1D nanomaterials have been synthesized for use insolar cells. Yang’s group17 prepared dye-sensitized solar cellscontaining ZnO nanowires, which increased the efficiency ofthe cell at higher dye loadings because of their large surfacearea. Tsakalakos et al.18 fabricated Si nanowire-based solar cellson metal substrates that exhibited enhanced optical propertiescompared with those of planar thin-film devices. Tang andcoworkers19 prepared photovoltaic cells containing CdS−Cu2Score−shell nanowires that showed excellent open circuitvoltage, fill factor, and response to low-light levels. However,it is still difficult to produce large-area nanowire arrays thatmight further increase the photoelectric efficiency of solar cells.
Increasing light absorption is an important way to improvethe performance and reduce the cost of photovoltaic cells.Diedenhofen’s group20 investigated the light absorption ofsemiconductor nanowire arrays with three different geometriesand found that layers of conical nanowires absorbed the mostlight. In contrast, the absorption relative to the volume fractionof semiconductor was the highest for layers of cylindricalnanowire arrays. Broadband, omnidirectional enhanced lightabsorption was observed for base-tapered nanowire arrays, withabsorption exceeding 97.5% in the wavelength range of 400−850 nm for a combined system of nanowires and substrate. Theabsorption in the nanowire layer alone is in the order of 80%,which is increased by almost 22% compared with thecorresponding film. This geometry can also reduce the amountof semiconductor material required in a device. Fan et al.21
investigated changes in the optical properties of highly orderedGe nanopillar arrays upon changing their shape and geometry.Ge nanopillar arrays with a diameter of 130 nm absorbed ∼85%of the incident light with minimal wavelength dependence for λ= 300−900 nm. Ge nanopillars with minimal reflectance andlarge diameter base exhibited 95−100% absorption for λ =300−900 nm, an increase of 18%. Most of the above
Received: November 2, 2012Revised: January 21, 2013Published: January 30, 2013
investigations are of nanowire arrays with very regulararrangement.The vapor−liquid−solid (VLS) method is commonly used to
prepare nanowire arrays to investigate their light absorptionability or application in solar cells. For example, InP nanowireshave been prepared by metal−organic vapor phase epitaxy,20
and Ge nanopillars have been prepared by a combination of theVLS method and electroplating.21 However, the VLS methodusually requires a metal catalyst, such as Au nanoparticles,which can contaminate the nanowires and increase leakagecurrents in devices. The nanowire arrays prepared by thesemethods are highly ordered.Cu2S is a p-type semiconductor with a bulk band gap of 1.21
eV and was one of the first semiconductors used to make solarcells. Cu2S nanowire arrays can be converted into copperindium gallium selenide (CIGS) or copper zinc tin sulfide(CZTS) nanowire arrays by further processing by electro-deposition or dipping in solution containing In, Ga, and Se ionsand subsequent annealing. This is because the single-crystalstructure of Cu2S nanowire arrays benefits from beingconverted to the perfect crystallinity of CIGS. Cu2S nanowirearrays have been synthesized by many methods.9−11,22−27
Among them, the solid−gas reaction is a cost-effective, easyscaled-up method to prepare large-area Cu2S nanowire arrayson Cu foil or films.9,10,28 This approach does not use anycatalysts or templates but simply uses mild reaction conditionsand gives high yields. In the solid−gas reaction, nanowire arraysare self-grown from a Cu substrate or Cu film, and the strongbonding between nanowires and the substrate ensures excellentelectrical contact between them. The Cu foil or film used as aprecursor promotes the growth of nanowire arrays and can alsobe used as an electrode and heat sink when the nanowire arraysare used in solar cells. We previously found that self-grownCu2S nanowire arrays enhanced light absorption29 but did notdetermine the full details of this relationship. We systematicallycontrol the morphology and distribution of Cu2S nanowirearrays to analyze their influence on light absorption.
2. EXPERIMENTAL DETAILS
Copper foil (99.9% purity) was carefully polished with number0−6 abrasive paper and then washed with deionized water. Thecopper foil was cleaned sequentially in an ultrasonic bathcontaining 3% sulfuric acid, then absolute ethanol, and finallydeionized water three times. The solid−gas reaction wasperformed in an airtight ceramic pipe. Pure O2 with a flow of800 sccm was introduced into the pipe for 5 min and thenswitched to the mixed gas of H2S (3000 sccm) and O2 (240sccm) for ∼3 min. The pipe was sealed and kept at a fixedtemperature of 18, 20, 25, 28, or 30 °C for 15 h. The area ofeach sample was about 15 × 25 mm.Scanning electron microscopy (SEM) was carried out using a
scanning electron microscope (Quanta 400F, FEI, France), andtransmission electron microscopy (TEM) was performed on anelectron microscope (JEOL JEM-2010HR) equipped with anenergy-dispersive X-ray spectrometer (EDS, Oxford, U.K.). X-ray diffraction (XRD) was performed on a diffractometer(Rigaku D/maxIIIA) with Cu−Kα radiation (λ = 0.154056nm). X-ray photoelectron spectroscopy (XPS) was performedon an X-ray photoelectron spectrometer (Escalab 250). Diffusereflectance spectra (DRS) were measured with a UV−vis-NIRspectrophotometer (UV-3150). Photoluminescence (PL) spec-tra were measured using a combined fluorescence lifetime andsteady-state spectrometer (FLS920). A Raman spectrum wasmeasured on a micro-Raman spectrometer (Renishaw inVia)using an excitation wavelength of 514.5 nm from an Ar ionlaser.
3. RESULTS AND DISCUSSION
The SEM images in Figure 1 show the structures of the Cu2Snanowire arrays. The morphology of each Cu2S nanowire arraydepends on the temperature used in the solid−gas reaction.The average diameter of each nanowire increases withtemperature, whereas the average length decreases. Forexample, the nanowire arrays prepared at 18 °C possessed an
Figure 1. SEM side-view images of Cu2S nanowire arrays and film. (S1) Cu2S film formed at 18 °C in 2 h and Cu2S nanowire arrays prepared at (S2)30, (S3) 28, (S4) 25, (S5) 20, and (S6) 18 °C for 15 h. (The scale bar is 2 μm.).
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average diameter of 114 nm and an average length of 8 μm,whereas those prepared at 30 °C have an average diameter of847 nm and an average length of 1.1 μm. The SEM images alsoshow that the nanowires are more or less deviated from thenormal direction of the substrate. We used the inclinationangle, which is the angle between the nanowires and the normaldirection of the substrate, to describe this deviation, as shownin Figure 1 (S1) and (S6), where the marked inclination angle is10 and 40°, respectively. The relationships between thepreparation conditions and the diameter, length, and spacingdensity of the Cu2S nanowire arrays are presented in Table 1.
The color of samples changed from black to gray as thenanowire diameter increased, and the Cu2S film was light gray,as shown in the photographs in Figure S3 in the SupportingInformation.An XRD spectrum of the nanowire arrays is presented in
Figure 2. The diffraction peaks can be indexed as monoclinic
Cu2S (JCPDS no. 33-0490). The relative intensity of the (2 ̅04)diffraction peak of the nanowire arrays increases the mostcompared with the intensity ratio in the standard PDF card ofCu2S, demonstrating that the {1 ̅02} crystal planes of the Cu2Snanowire arrays preferentially align parallel to the surface of thesubstrate.EDS indicates that the nanowires are composed of Cu, S, and
O. The atomic ratio of Cu to S is greater than 2 because of theadditional copper signal coming from the Cu substrate (FigureS4 in the Supporting Information).TEM and HRTEM observations (Figure 3) indicate that the
growth direction of Cu2S nanowire is [1 ̅02], consistent withXRD results. The images also clearly show that the surface of
the nanowires is covered with a thin layer of CuxO (∼4 nmthick) to form a core/shell nanowire structure. The latticespacing perpendicular to the radial direction of a nanowireobtained from a HRTEM image (Figure 3b) of the core of thenanowire is ∼0.67 nm and is 1.345 nm along the radialdirection, corresponding to the interplanar distances of the(1 ̅02) and (100) crystal planes of monoclinic Cu2S,respectively. The lattice spacing of the shell part is ∼0.302nm, corresponding to the interplanar distance of the (110)crystal plane of cubic Cu2O, but most of the Cu2O on thenanowire surface has low crystallinity. The selected-areaelectron diffraction (SAED) pattern of this region of thenanowire (inset in Figure 3b) confirms the presence of Cu2Oand Cu2S. This finding is also consistent with a Ramanspectrum (Figure 4) and O 1 s and Cu 2p XPS (Figure 5) ofthe Cu2S nanowire arrays.
Raman spectrum and XPS also confirm that the shell of thenanowire contains both CuO and Cu2O. Raman peaks at 219,298, 407, and 625 cm−1 are attributed to Cu2O and CuO,30−32
and a peak at 472 cm−1 corresponds to Cu2S in the core of thenanowire.28,33,34 Comparing Figure 5a,b reveals that the relativeamounts of both CuxO and Cu2S increase as the Cu2S nanowiregrows (Table S1 in the Supporting Information).PL spectra of the Cu2S nanowire arrays (Figure 6) show that
their band gap varies between 1.251 and 1.272 eV. As the
Table 1. Relationship between Preparation Conditions andthe Diameter, Length, and Density of Cu2S Nanowire Arrays
Figure 3. (a) TEM image of Cu2S nanowire and (b) HRTEM imageof the region marked in panel a. The inset shows the SAED pattern forthis region.
Figure 4. Raman spectrum of Cu2S nanowire arrays prepared at 18 °Cfor 15 h (S6).
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diameter of the nanowires increases from 114 to 847 nm, theband gap first decreases from 1.266 to 1.251 eV and thenincreases to 1.272 eV. In contrast, the full width at half-maximum (fwhm) of the PL peaks reflects the amount of CuxOon the surface of the nanowires. The inset in Figure 6 showsthat the fwhm first decreases from 0.14 to 0.026 eV when thenanowire diameter increased from 114 to 425 nm. It thenincreased to 0.151 eV when the diameter increased to 847 nm,indicating that the amount of CuxO on the surface of thenanowires first decreases and then increases as the nanowirediameter increases. The PL experiments show that the band gapand amount of CuxO change in the same manner with respectto nanowire diameter, first decreasing and then increasing asthe nanowire diameter increases. There are two possible
reasons for this phenomenon: one is that the diameter of thenanowires influences both the band gap and the thickness ofCuxO film, and the other one is that the thickness of CuxO filminfluences the band gap of the Cu2S nanowires. However, thedetailed relationship between the thickness of CuxO film, theband gap, and the diameter of Cu2S is still remainedinvestigation. The above results also indicate that the bandgap of Cu2S nanowire arrays can be adjusted by changing themorphology and size of the nanowires. This is anotheradvantage for Cu2S nanowire arrays in photovoltaic or heatingharvesting applications.We systematically measured the light-absorption properties
of Cu2S nanowire arrays. DRS of the nanowire arrays withdifferent diameters and morphologies are presented in Figure7a1. When the incident light is along the normal direction of thesample surface, all Cu2S nanowire arrays show good lightabsorption over a wide range of wavelengths from 240 to 800nm, including visible, UV, and infrared regions. The nanowirearrays absorb light more strongly than the Cu2S thin film. Theaverage light absorption increases by ∼34% as the nanowirediameter decreases, from 70% for the Cu2S film (S1 curve inFigure 7a1) to 93.5% for the Cu2S nanowires with a diameter of114 nm (S6 curve in Figure 7a1), at light wavelength of 433 nm,the light absorption values even increases by 43%, and theenhanced light absorption abilities remain almost constant(about 92−95%) over a wide range of wavelengths from 240 to670 nm, presenting a very strong enhanced light absorption.The light absorption properties of ordered nanowire arrays of
different sizes and shapes have been investigated extensively, forexample, Chen’s work on silicon nanowire arrays withdiameters between 50 and 80 nm and lengths of 1.16−4.66μm35 and Fan’s study of Ge dual-diameter nanopillars with tipsof small diameter (D1 = 60 nm), base of large diameter (D2 =130 nm), and length of 2 μm.21 In these reports, nanowire
Figure 5. XPS of Cu2S nanowire arrays. (a) O 1s XPS and Cu 2p XPS of Cu2S nanowire arrays prepared at 18 °C for 15 h (S6) and (b) O 1s XPS,Cu 2p XPS, and S 2p XPS of the Cu2S film grown at 18 °C for 2 h (S1).
Figure 6. PL spectra of the as-prepared Cu2S nanowire arrays. Theinset shows the relationships of the diameter of the Cu2S nanowiresversus band gap and fwhm of PL.
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arrays were very regular vertical arrays with more even size andspacing distribution than the nanoarrays produced here. Thelight absorption of silicon nanowire arrays is sensitive towavelength: higher in the high-frequency regime and lower in
the low-frequency regime. In addition, the larger the diameterof the nanowires, the higher its absorption for wires of the samelength. In the high-frequency regime, nanowires absorb morelight than optically dense thin films. In the low-frequency
Figure 7. (a1) DRS of Cu2S nanowires with different diameters. (a2) and (a3) show DRS with different light incidence angles for S2 and S6 Cu2Snanowire arrays, respectively. (b1) and (b2) show the relationships between the diameter, length and average absorbance of samples. (b3)Dependence of absorbance on material filling ratio, which is defined as the occupation of the cross section of a Cu2S nanowire array per 1 μm
2 area.
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photon regime, films absorb more efficiently than nanowirearrays, which can be improved by increasing the length of thenanowire because of the small extinction coefficient of silicon.This means that the tip portion of the Ge dual-diameternanopillars with small diameter has low reflectance, leading toefficient photon trapping and transmission down to the base.Meanwhile, the base layer with large diameter has a highmaterial filling ratio and thus can efficiently absorb light; sucharrays exhibit 95−100% absorption for λ = 300−900 nm.In our work, light absorption can reach 91−95% over the
whole wavelength range from 240 to 800 nm for the nanowiresof smaller diameter, even though our Cu2S nanowires have aslightly uneven distribution and much larger diameters thanthose discussed above. This may be related to their largerextinction coefficient36,37 and the distinct distribution andgeometry of these nanowire arrays.Figure 7a2,a3 shows DRS measured at different light incident
angles (0, 10, 20, and 40°) for Cu2S nanowire arrays withdifferent morphologies and diameters over the wavelengthrange of 200−850 nm. The nanowires with bigger diameters(847 nm) are relatively evenly distributed (Figure 1 (S2)), andhave small inclination angles (<10°). The light absorption ofthese nanowires (Figure 7a2) is the highest at an incident angleof 10°. The nanowires of smaller diameter (114 nm) have anuneven distribution (Figure 1 (S6)). The inclination angles ofmost of these nanowires are <40°, with the highest probabilitywithin 0−10°, but the amount of nanowires with inclinationangles of 10−20, 20−30, and 30−40° is almost equal (FigureS2 in the Supporting Information). They absorb the most lightat an incident angle of 0° (Figure 7a3), and the absorptionvaries less at other incident angles than was observed for thenanowires with larger diameters. Zhu et al.38 found that whenthe incident angle of light increases, reflection also increases,resulting in a decrease in absorption. In our work, because ofthe slight inclination angle of the nanowires, the lightabsorption ability of Cu2S nanowire arrays is less sensitive tothe incident angle of light. The nanowires with bigger diameterabsorb more light at an incident angle of 10° than at an incidentangle of 0°, but the average light absorption increases by only3% for incident angles of 0−10° and decreases by 9% forincident angles of 10−40° in the wavelength range of 400−800nm. For the Cu2S nanowires with smaller diameter, the lightabsorption between 240 and 670 nm decreases by only 3%when the incident angle was increased from 10 to 40°. Theseare much better comprehensive light-absorption propertiescompared with those of very regular nanowire arrayssynthesized by the VLS method.Figure 7b1 shows the relationship between nanowire
diameter and the average light absorption between 250 and800 nm. (The noise in the spectrum at this wavelength iscaused by external factors and has a small fluctuation of ∼5%.)The observed linear relationship clearly demonstrates that theabsorbance is primarily related to the diameter of thenanowires. Cu2S nanowires of large diameter show increasedreflection and reduced absorption compared with those ofsmaller diameter. Figure 7b2 shows the relationship betweennanowire length and light absorption, which can be divided intotwo regions with a linear relationship. First, the rate of increasein absorption is sharp, following the increase in nanowire lengthup to 1.5 μm. The rate of increase is lower as the length exceeds1.5 um, which may also be related to the inclination distributionof the Cu2S nanowire arrays. As the length of the nanowiresincreases, so does the inclination angle of the nanowires,
increasing reflection. The dependence of absorption on thematerial filling ratio, which is defined as the occupation of thecross-sectional area of Cu2S nanowire array per 1 μm2, ispresented in Figure 7b3. The light absorption first increases andthen decreases as the filling ratio increases, which probably isascribed to changes in both the length and diameter of theCu2S nanowires.
4. CONCLUSIONSIn conclusion, self-grown single-crystalline Cu2S nanowirearrays were prepared by the solid−gas method. Their surfaceabsorption of light and band gap can be adjusted by changingthe morphology and distribution of nanowires in the arrays.The nanowire arrays absorb light more strongly than a Cu2Sthin film, and the average light absorption increases as thenanowire diameter decreases. An increase in absorption of∼34% was observed from the Cu2S film to the array of Cu2Snanowires with an average diameter of 114 nm for a wide rangeof wavelengths; the light absorption even increases by 43% atthe light wavelength of 433 nm. The light absorption almostremains constant (about 92−95%) in a wide range of lightwavelength between 240−670 nm, presenting a very strongenhanced light absorption. The absorption of the Cu2Snanowire arrays is less sensitive to both wavelength andincident angle than that of films. The average light absorptionof Cu2S nanowires with smaller diameter decreases only slightlyas the incident angle increases from 10 to 40° between 240 and670 nm. The band gap of Cu2S nanowire arrays can be adjustedby changing the morphology of the nanowires, which wasachieved simply by changing the temperature during theirformation. Our results indicate that self-grown Cu2S nanowirearrays show potential for application in solar energy.
■ ASSOCIATED CONTENT*S Supporting InformationDevice preparation, photographs of samples, proportion ofCu2S nanowires with different inclination angles, and contentratios of ions with different valences and EDS of Cu2Snanowires. This material is available free of charge via theInternet at http://pubs.acs.org.
■ ACKNOWLEDGMENTSThis work was partially funded by the Science & TechnologyCommittee of Guangdong Province, China (project no.2010A080805002).
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