Letter pubs.acs.org/NanoLett
GaP−ZnS Pseudobinary Alloy Nanowires Kidong Park,† Jung Ah Lee,† Hyung Soon Im,† Chan Su Jung,† Han Sung Kim,† Jeunghee Park,*,† and Chang-Lyoul Lee‡ †
Department of Chemistry, Korea University, Jochiwon 339-700, Korea Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea
‡
S Supporting Information *
ABSTRACT: Multicomponent nanowires (NWs) are of great interest for integrated nanoscale optoelectronic devices owing to their widely tunable band gaps. In this study, we synthesize a series of (GaP)1−x(ZnS)x (0 ≤ x ≤ 1) pseudobinary alloy NWs using the vapor transport method. Compositional tuning results in the phase evolution from the zinc blende (ZB) (x < 0.4) to the wurtzite (WZ) phase (x > 0.7). A coexistence of ZB and WZ phases (x = 0.4−0.7) is also observed. In the intermediate phase coexistence range, a core−shell structure is produced with a composition of x = 0.4 and 0.7 for the core and shell, respectively. The band gap (2.4− 3.7 eV) increases nonlinearly with increasing x, showing a significant bowing phenomenon. The phase evolution leads to enhanced photoluminescence emission. Strikingly, the photoluminescence spectrum shows a blue-shift (70 meV for x = 0.9) with increasing excitation power, and a wavelength-dependent decay time. Based on the photoluminescence data, we propose a type-II pseudobinary heterojunction band structure for the single-crystalline WZ phase ZnS-rich NWs. The slight incorporation of GaP into the ZnS induces a higher photocurrent and excellent photocurrent stability, which opens up a new strategy for enhancing the performance of photodetectors. KEYWORDS: GaPZnS nanowires, quaternary composition tuning, pseudobinary, wurtzite,-zinc blende phase evolution, band gap
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The GaP−ZnS system is known to form a solid solution with all four elements in the full compositional range.34 Interestingly, however, the band gap of the solid solution was near to that of pure GaP across a very wide composition range (up to ∼70 at. % ZnS).35,36 Recently, Hart and Allan performed ab initio calculations to explain the nonlinear dependence of the band gap on composition.37 Remarkably, they predicted that the addition of a small amount of ZnS to GaP (or vice versa) produces a semiconductor with a significantly smaller band gap than pure GaP (or ZnS), bringing the direct band gap into the visible-light range. However, there are few works on the full compositional tuning of NWs that have different properties from the bulk phase. In the present work, we show the synthesis and compositional tuning of (GaP)1−x(ZnS)x pseudobinary alloy NWs using a simple vapor transport method. The composition tuning was successfully achieved by changing the ratio of GaP and ZnS powder. The band gap (Eg) could be tuned between the UV and visible regions (Eg = 2.4−3.7 eV). Compositional tuning resulted in a phase evolution from the zinc blende (ZB) (x < 0.4; GaP-rich alloy) to the wurtzite (WZ) (x > 0.7; ZnS-rich alloy) phase. In the intermediate composition range (x = 0.4−
ulticomposition alloying of semiconductors offers the advantages of band gap tunability, controlled conduction band gap offsets, and localized defect energy levels, which are critical for achieving high photoconversion efficiencies in photovoltaic cells. Nanowire structures have attracted considerable attention because they can be used as well-defined building blocks of future nanodevices with unique optical and electrical properties using bottom up approaches. Achieving compositionally tuned homogeneous alloy NWs is challenging because of the inherent differences in precursor reaction kinetics, which require adept control of the reactivity of individual precursors. Nanowires with a ternary or quaternary composition in III−V semiconductors (e.g., InGaAs,1−7 InGaN,8 AlGaAs,9 GaInSb,10 GaAsP,11−14 AlGaP,12 InAsP,15 and InAsSb16) or II−VI, IV−VI (e.g., CdSSe,17−23 ZnCdS,24,25 ZnCdSe,26,27 ZnSSe,28,29 PbSSe,30 ZnCdSSe,31 etc.) have given rise to remarkable discoveries. Nevertheless, the synthesis of NWs with an attractive quaternary composition in a pseudobinary system comprised of III−V and II−VI semiconductors remains a great difficulty. Although the properties of such NWs can be tailored through a wide compositional range, the physical properties as a function of composition remain unknown. Recently, Liu et al. reported the synthesis of (GaP)1−x(ZnS)x NWs at a composition x = 0.045 and 0.968.32 The Pan group have reported (GaAs)1−x(ZnSe)x NWs with compositions in the range of x < 0.5.33 © 2014 American Chemical Society
Received: July 28, 2014 Revised: September 6, 2014 Published: September 18, 2014 5912
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peak. We resolved the peak by two Voigt functions (P1 and P2). The composition of P1 (red color) and P2 (black color) bands was x1 = 0.4 and x2 = 0.7, respectively, assuming a ZB phase. The sum of two bands is displayed by the gray color line, which fits well the experimental data. If the P1 band had an overlap with the (002) peak of the WZ phase, a value of x1 = 0.7 was found for the WZ phase using the reference values for bulk ZnS (JCPDS Card No. 36-1450; a = 3.820 Å and c = 6.257 Å) and GaP (a = 3.8419 Å and c = 6.3353 Å).39 As shown in the following TEM images and EDX data, the NW consisted of a GaP-rich core and ZnS-rich shell. In the ZnS-rich shell, the ZB and WZ phases coexist. Therefore, we conclude that the ZB phase core has a composition of x = 0.4 and the ZB-WZ phase shell has a composition of x = 0.7. This suggests that excess ZnS produces the ZnS-rich alloy (x = 0.7) owing to the limited miscibility of the GaP-rich alloy. The x = 0.7 and 0.9 compositions exhibit peaks corresponding to the WZ phase. The ZnS NWs exhibit both WZ and ZB phases, with (111) and (002) peaks resolved using Voigt functions. In summary, the GaP-rich alloy (x < 0.4) NWs remain in the ZB phase, whereas the ZnS-rich alloy (x > 0.7) NWs exist in the WZ phase. A maximum of 40 at. % ZnS is miscible in the ZB phase of GaP. A maximum of 30 at. % GaP is miscible in the WZ phase of ZnS and induces the phase transition into the ZB phase. NWs with intermediate composition consisted of two saturated compositional alloys of x = 0.4 and 0.7 for the core and shell, respectively. Figure 2a shows SEM images of high density NWs grown on substrates. The HRTEM image of a selected (GaP)0.9(ZnS)0.1 NW (diameter = 200 nm) reveals its single crystalline ZB phase (Figure 2b). The NWs exhibited straight and smooth surfaces without any amorphous outer layers. The inset shows fast Fourier transform (FFT) images along the [011̅] zone axis, confirming a single crystalline NW grown along the [111] direction. The d spacing between the neighboring (111) planes is 3.1 Å, which is consistent with that of x = 0.1 (Cf. dGaP = 3.146 Å and dZnS = 3.086 Å). EDX data confirms the composition (Supporting Information, Figures S1 and S2). In all cases for x = 0.4−0.7, the core−shell structure developed along the entire wire axis (Figure 2c). The surface of the NWs is zigzagged. The diameter of the core and shell is about 20 and 60 nm, respectively. EDX line scan profiles (along the cross section) and elemental mapping of a typical (GaP)0.5(ZnS)0.5 NW show Ga (K or L shell) and P (K shell) elements predominantly in the core, and Zn (K or L shell) and S (K shell) elements predominantly in the shell (Figure 2d). A scanning TEM (STEM) image is also displayed. The ratio of Ga:P and Zn:S is 1:1. The ratio of GaP and ZnS is ∼6:4 and ∼3:7, respectively, for the core and shell. Therefore, we conclude that the NW consists of a (GaP)0.6(ZnS)0.4 core and (GaP)0.3(ZnS)0.7 shell, based on the EDX and XRD data. Figure 2e shows HRTEM and corresponding FFT images for the ZB-WZ polytypic structure of the shell part (at the zone axis of ZB [01̅1̅] and WZ [21̅1̅0]). We denote the phase using the subscripts. The wire axis matches with the ZB [111] and WZ [0001] direction. The d spacing between the neighboring ZB (111) and WZ (002) planes is 3.1 Å, which is consistent with the reference values (Cf. c = 6.3353 Å for GaP and c = 6.257 Å for ZnS). This polytypic structure contains twinned superlattice segments having various sizes. In the FFT image (inset), the zigzagged segments share the ZB [111] and [1̅1̅1̅] spots (see the twin spots marked in red/yellow color). In the bright/dark striped region, the superlattices have a periodicity
0.7), the GaP-rich (x = 0.4) and ZnS-rich alloys (x = 0.7) coexist. The nature of compositional-dependent nanostructures was thoroughly examined using high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), ultraviolet (UV)-visible absorption, and photoluminescence (PL) spectroscopy. Recently, GaP−ZnS core−shell heterostructure NWs have been demonstrated to be useful in a wide range of potential applications as photodetectors.38 The present photocurrent measurements on individual NWs indicate that a slight incorporation of GaP into the WZ phase ZnS could lead to the abrupt enhanced performance of photodetectors. Results and Discussion. The growth of the alloy NWs on Au coated Si substrates follows the typical vapor−liquid−solid (VLS) mechanism, which makes use of Au nanoparticles as catalysts for growth. A total of 10 NW samples were prepared. EDX data of the Au nanoparticle tip provides an evidence for the VLS mechanism, as shown in the Supporting Information, Figure S1. XRD patterns over the full 2θ range of the (GaP)1−x(ZnS)x NWs were acquired, as shown in Figure 1a. As x increases, the
Figure 1. (a) XRD pattern over the full 2θ range and (b) magnified ZB (111) (or WZ (002)) peak of (GaP)1−x(ZnS)x (x = 0, 0.1, 0.35, 0.5, 0.7, 0.9, and 1) NWs. The WZ (red color) and ZB (black color) peaks are indexed using reference peaks from the appropriate JCPDS card (see text).
peak position shifts continuously from the GaP ZB phase to the ZnS WZ phase. The ZB (111) and WZ (002) peaks are displayed over a magnified scale in Figure 1b. The peaks of GaP (x = 0) NWs were assigned to the ZB phase of GaP (JCPDS Card No. 32-397; a = 5.450 Å). For x = 0.1 and 0.35, the shifted peaks could be assigned to the ZB phase alloy. The composition (x) was determined using Vegard’s law (i.e., d = (1 − x)dGaP + xdZnS) based on the peak position of the ZB phase ZnS (JCPDS Card No. 80-0020; a = 5.345 Å), and was found to be consistent with the EDX data that shows the 1:1 ratio of Ga:P and Zn:S (see Supporting Information Figures S1 and S2). For x = 0.5, both WZ and ZB peaks appeared. A significant peak broadening was observed for the ZB (111)/WZ (002) 5913
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and the ZB-WZ polytypic structure at the larger diameter shell part. Figure 3a shows the HRTEM and the corresponding FFT images (zone axis = [0001]WZ) of (GaP)0.1(ZnS)0.9 NWs
Figure 3. HRTEM and FFT images of a selected (GaP)0.1(ZnS)0.9 NW, at the zone axis of (a) [0001]WZ and (b) [211̅ 0̅ ]WZ, showing a single crystalline WZ phase with a growth direction of [01̅10]. HRTEM and FFT (zone axis = [011̅ ]̅ ZB/[211̅ 0̅ ]WZ) images of (c) ZnS NW and (d) NB, showing their ZB-WZ polytypic structures. Figure 2. (a) SEM image of high density NWs homogeneously grown on a substrate. (b) HRTEM and corresponding FFT images (inset) of a selected (GaP)0.9(ZnS)0.1 NW at the [011̅]ZB zone axis show a single crystalline feature with a growth direction of [111]. (c) HRTEM image for (GaP)0.5(ZnS)0.5 core−shell NWs. (d) EDX line scan and mapping (with STEM image) reveal a GaP-rich core and ZnS-rich shell. (e) Lattice-resolved image and corresponding FFT images (inset) for the shell (growth direction = [111]ZB and [0001]WZ) reveal a polytypic twinned superlattice structure along the [011̅]ZB (and [21̅1̅0]WZ) zone axis. (f) When tilted by 30°, the HRTEM and corresponding FFT (zone axis = [112̅]ZB and [011̅0]WZ) images show single crystalline features without twin planes.
(diameter = 300 nm). The images reveal a single crystalline WZ phase with a growth direction of [0110̅ ]. Along the zone axis of [21̅1̅0], there are no extra spots or lines along the [0001] directions (perpendicular to the growth direction), indicating the absence of polytypic defects (Figure 3b). EDX line-scan data show that the elements distribute homogeneously over the entire NW (see Figures S1 and S2 in the Supporting Information). The d spacing between neighboring WZ (002) planes is 3.1 Å, which is consistent with the value of x = 0.9. Figure 3c shows the HRTEM and corresponding FFT (zone axis = [011̅]ZB/[21̅1̅0]WZ) images of the ZnS NW (diameter = 50 nm). It consisted of the ZB-WZ polytypic structures along the [0001] growth direction. The d spacing between neighboring ZB (111) or WZ (002) planes is 3.1 Å, which is consistent with the values expected for bulk ZnS. The ED spots form the lines along the [111]ZB/[0001]WZ growth direction owing to the polytypic structures (inset). When the diameter is larger than 50 nm, the morphology becomes the nanobelts (NBs). The ZnS NBs are grown along the [01̅10] direction, as shown in Figure 3d. The HRTEM and FFT images confirm the ZB and WZ domains that aligned parallel to the growth direction. The UV−visible diffuse reflectance spectrum was measured for all NW samples (as grown on a silicon substrate) prepared in this study. Figure 4a displays the UV−visible spectrum for x = 0, 0.1, 0.35, 0.5, 0.7, 0.9, and 1. The compositional tuning enabled the band gap to show absorption over a wide range of 300−850 nm (corresponding to 1.5−3.7 eV). The Kubelka− Munk (K−M) transformation, [F(ν)hν]1/2 (where F(ν) is the diffuse reflectance spectrum) yielded the indirect band gap. Figure 4b shows a K−M plot of [F(ν)hν]1/2 versus photon energy (hν). Photoluminescence (PL) measurements (at 8 K) were performed by delivering continuous wave excitation from
of 0.9 nm, which coincides with three times the ZB (111) or WZ (002) planes. One possible arrangement is that two bright (111) planes of the ZB structure and one dark (002) plane of the WZ structure are stacked alternately. Therefore, the WZ phase diffraction spots (marked in green color) are clearly identified between the ZB phase twin spots. The TEM grid holder was rotated to tilt the NW around the axial direction. Figure 2f shows the HRTEM image (zone axis = ZB [112̅]/WZ [011̅0]), at a tilt angle of 30°, which exhibits single crystalline features without any twin planes. This unique set of sequential changes confirmed that the NW consisted of octahedral slice segments with alternate orientations along the axial [111] direction. The formation of two saturated alloy phases; x = 0.4 (ZB) and 0.7 (ZB/WZ), results in the core−shell heterostructures. A number of studies on the III−V and II−VI semiconductor NWs showed that the effectively reduced energy barrier for the nucleation within the smaller diameter catalysts predominantly favors the pure cubic phase.40−42 The increase of diameter induces the ZB-WZ polytypic structures. Therefore, the ZB phase are grown preferentially at the smaller diameter core part, 5914
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the WZ phase ZnS-rich alloy, which depends significantly on composition. A nonlinear feature (known as a bowing phenomenon) is consistent with the previous studies in films in which Eg was close to that of pure GaP over the range x = 0−0.7 and increases steeply afterward.35,36 We observed that a little incorporation (x = 0.1) of ZnS in the GaP leads to a decrease of 0.1 eV. The incorporation of GaP in ZnS (x = 0.9) decreases Eg by 1.0 eV. According to the calculations of Hart and Allan, the band gap of (GaP)0.875(ZnS)0.125 and (GaP)0.125(ZnS)0.875 can be reduced to 1.9 and 2.5 eV, respectively, which is much less than that of pure GaP (2.47 eV) and ZnS (3.41 eV).37 Therefore, the decrease of Eg upon incorporation of ZnS in GaP or vice versa is consistent with the theoretical work. The incorporation of ZnS in the GaP enhances the band edge emission (at ∼2.5 eV), suggesting the change from the indirect band gap to the direct band gap. It is probably due to the direct band gap properties of the ZnS that exists as pseudobinary alloy component. All alloy NWs exhibited a blue shift in the PL spectrum with increasing pump power. In contrast, the GaP and ZnS NWs did not exhibit an observable blue-shift. As shown in Figure 5a, the
Figure 4. (a) UV−visible diffuse reflectance spectrum (in absorption), (b) K−M plot ([F(ν)hν]1/2), and (c) PL spectrum (measured at 8 K) of (GaP)1−x(ZnS)x NWs, where x = 0, 0.1, 0.35, 0.5, 0.7, 0.9, and 1. The excitation source is a He−Cd laser (325 nm, power = 22 mW). (d) Dependence of the band gap on the ZnS content (x), determined by the onset of the UV−visible spectrum, K−M plot (indirect), and band-edge peak (PL).
a 325 nm He−Ne laser to the NW samples. The PL emission intensity increases with increasing x. Figure 4c displays the PL spectrum over a normalized scale. The optical band gap (Eg) was determined by the peak position of the band edge emission and plotted as a function of x (Figure 4d). The onset of UV− visible absorption and the indirect band gap (K−M plot) were also plotted. For GaP and ZnS NWs, the onset of UV−visible absorption and the band edge emission appear at 2.4 and 3.6 eV, respectively, which is consistent with the band gap of the bulk (2.4 and 3.7 eV). The PL spectrum of ZnS NWs exhibits two broad emission bands at 3.2 and 2.7 eV, which may originate from the defect sites formed at the interface of the WZ and ZB domains. The GaP NWs exhibit the strong emission at 1.7 eV, which is also originated from the defect sites. For the GaP-rich alloy (x < 0.4) NWs, the absorption onset shifts to a lower energy relative to that for GaP NWs. The x = 0.1 and 0.35 compositions exhibit a weak band edge emission at 2.2 and 2.4 eV (marked by arrows), respectively. The broad emission at ∼1.6 eV is ascribed to their alloy phase that enhances the nonradiative quenching process through many defect sites. The core−shell NW (x = 0.5) shows the band edge emission at 2.3 eV, which is the same as that for the indirect band gap. For the ZnS-rich alloy (x = 0.7 and 0.9) NWs, the peak position shifted rapidly to higher energy (2.4 and 2.6 eV, respectively) with increasing x, accompanying a significant intensity increase. The width of the band edge emission is broader than that of the ZnS NWs. The Eg values are nearly consistent with the onset energy and the indirect band gaps (obtained by the K−M plot). The x < 0.2 exhibits a slightly lower Eg (2.2−2.3 eV) than the GaP NWs. The Eg of x > 0.7 increases dramatically with increasing x. The same Eg of the core−shell NWs (x = 0.5 and 0.6) as that of the GaP-rich alloy (x < 0.4) NWs may be due to the (GaP)0.6(ZnS)0.4 composition, which probably has a lower band gap relative to (GaP)0.3(ZnS)0.7. As the (GaP)0.6(ZnS)0.4 absorption disappeared, Eg could be determined by that of
Figure 5. (a) PL spectrum of (GaP)0.1(ZnS)0.9 NWs (measured at 8 K), measured under different excitation laser intensities (power in mW). The spectrum is normalized to the same scale for ease of comparison. The integrated PL intensity versus excitation power shows a linear increase (inset). (b) PL band-edge peak (in eV) dependence on the excitation power (at 8 K) for x = 0.35, 0.5, 0.7, and 0.9. (c) PL decay curves of (GaP)0.1(ZnS)0.9 NWs at representative different emission energies under low excitation power (0.2 mW). (d) A schematic diagram for the type-II band structure of WZ phase ZnSrich alloy NWs. The three carrier recombination paths (GaP-like and ZnS-like band edges and interlayer recombination) are illustrated.
band-edge peak of (GaP)0.1(ZnS)0.9 NWs shifts from 2.51 to 2.58 eV. The shift is about 70 meV over an approximately 100fold increase in pump power (0.2−22 mW). Figure 5b displays the peak position (at 8 K) as a function of excitation power, for x = 0.35, 0.5, 0.7, and 0.9. The NWs with higher x exhibit a larger blue shift; 20, 20, and 40 meV for x = 0.35, 0.5, and 0.7, respectively. We observed a more significant blue shift (about three times larger) at room temperature (see Supporting 5915
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ZnS NWs would be an indication of a wide distribution of GaP−ZnS pseudobinary layers. We refer to these GaP and ZnS layers as GaP-like and ZnS-like layers, respectively. Herein, we propose a new type-II band structure, a pseudobinary heterojunction. Figure 5d illustrates the prevailing staggered band structure of the pseudobinary heterojunction that consists of GaP-like and ZnS-like layers, with a type-II band offset. Upon laser excitation, the generated photoelectrons are confined to the GaP-like layers and holes are separated at the ZnS-like layers with a longer lifetime. The interlayer emission occurs at longer wavelengths, following electron−hole recombination. Under a higher pump power, more carrier charges accumulate at the interface, thus enhancing the band bending. Photoexcited electrons start to occupy the higher energy conduction band (CB) of ZnS-like layers, producing the higher emission energy. As x increases, the effective potential well width is then reduced so as to raise energy levels, causing a more blue shift in the PL emission as pump power increases. The dependence of the PL decay time on the emission energy (at the low pump power) supports the heterojunction layers having different carrier lifetimes. The emission from the CB of GaP-like layers occurs at shorter wavelengths than that of interlayer emission, with a faster radiative recombination. Recently, the Bakkers group reported 0.78 ns for a PL decay time of WZ phase GaP NWs, which is much shorter than that of ZB phase GaP NWs (254 ns at 2.3 eV).12 The shorter decay time component (0.8 ns) is nearly the same as that of the WZ phase GaP NWs. This result indicates that the GaP-like layers have consistently direct band gap properties of the WZ phase GaP. Furthermore, the emission at longer wavelengths has a long decay time of 1 μs, which is even longer than that of ZB phase GaP NWs. Therefore, the long lifetime should be correlated with interlayer emission. Consequently, the GaP-like and ZnS-like heterojunction layers have a pronounced effect on photocurrents, as described below. We fabricated a photodetector device using DEP and FIB techniques. Figure 6a shows the current−voltage (I−V) curves
Information, Figure S3). The corresponding total integrated PL intensity shows no sign of saturation (inset of Figure 5a). The linearity of the alloy NW (even at room temperature) indicates that the optical transition responsible for the PL is not impurity related. We measured the time-resolved PL spectrum as a function of composition (Supporting Information, Figure S4). Figure 5c displays the PL decay curves monitored at various emission energies of 2.3, 2.6, 2.75, 2.8, and 3.0 eV (for wavelength range 420−540 nm), under a low pump power (0.2 mW), for (GaP)0.1(ZnS)0.9 NWs. The excitation photon energy is 3.3 eV (375 nm was obtained from picosecond pulsed diode laser). The decay curve shows a dramatic increase in lifetime for the lower energy emission. The decay curve for 3.0 eV emission was fitted with a double exponential decay function. The average decay time ⟨τ⟩ was calculated using the equation, ⟨τ⟩ = α1τ1 + α2τ2, where αi is the amplitude and τi is the decay time, yielding ⟨τ⟩ = 2.6 ns with τ1 = 0.8 ns (75%) and τ2 = 8 ns (25%). For the 2.3 eV emission, ⟨τ⟩ is increased to be 1 μs (the full decay curve is shown in the Supporting Information, Figure S5). The decay curves of (GaP)0.5(ZnS)0.5 NWs showed ⟨τ⟩ = 9 and 90 ns at 2.6 and 2.3 eV, respectively (see Supporting Information, Figure S6). Under this low pump power, the PL emission of the GaP and GaP-rich NWs were too weak for decay curve measurements, which implies very short decay times. The decay curve of ZnS NWs showed negligible emission energy dependence with ⟨τ⟩ = 1 ns. Overall, the power dependent blue shift and the wavelength dependent decay time become more significant for higher x (only alloy). This observation suggests that the WZ phase ZnS-rich alloy has multiple possible band structures (at least two), in which the higher excited states with faster nonradiative quenching pathways are more populated under higher excitation power. A power dependent blue shift in PL has been observed in III−V semiconductors such as Ga0.5In0.5P (layers), and InP, InAs, and GaAs/GaAsP (core/shell) NWs.43−47 This effect is usually observed for type-II band structures of ZB and WZ phases. The coexistence of two phases is often manifested by forming sequential polytypic twinned superlattices structures along the ZB [111] growth direction.46−50 Photoexcited electrons and holes begin to occupy the higher energy subbands under a high pump power and band filling occurs, resulting in both high emission energy and transition probability. We now consider a possible band structure based on the ZB-WZ microstructure of the present (GaP)1−x(ZnS)x alloy NWs. The direct band gap of the WZ phase of ZnS is slightly larger (∼0.1 eV) than that of the ZB phase, with a typeII band offset.51 The band structure of GaP and corresponding band offset are still debatable. The calculations predicted ∼0.1 eV smaller values for the direct band gap of WZ GaP, with a type-I or -II band ordering.52,53 Recently, the Bakkers group reported the PL spectrum of the WZ phase of GaP NWs at 2.09 eV.12 Therefore, there is no guarantee of forming a type-II ZBWZ band alignment for the (GaP)1−x(ZnS)x alloy. This explains why a smaller blue shift and wavelength dependent lifetime were observed for core−shell NWs, where the twinned superlattice structures exist in the shell. As we stated above, the blue shift could originate from the WZ phase Zn-rich alloy. Hart and Allan showed that the incorporation of GaP layers into the ZnS layers decreased the band gap by decreasing the number of Zn−S bonds.37 The broader PL emission of the alloy NWs relative to that of the
Figure 6. (a) I−V characteristics of (GaP) 0.9 (ZnS) 0.1 and (GaP)0.1(ZnS)0.9 NWs under 365 nm (100 mW) irradiation and under dark conditions. The SEM image shows the NW aligned between Ti/Au bottom electrodes with a 1.5 μm gap, with a deposition of Pt top electrodes using FIB (inset). (b) I−t curves at a bias voltage of 5 V under chopped irradiation.
of the device using (GaP)0.9(ZnS)0.1 and (GaP)0.1(ZnS)0.9 NWs, under dark conditions and under irradiation at 365 nm (2.4 eV) using a diode laser (10 mW). SEM shows the NW aligned between Ti/Au bottom electrodes with a gap of 1.5 μm (Inset). The distance between the Pt top electrodes is approximately 10 μm. The average size of NWs is 200 nm 5916
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Materials and Methods. ZnS (99.99%, Sigma-Aldrich) and GaP (99.99%, Sigma-Aldrich) powders were placed separately in two quartz boats, which were loaded inside a quartz tube reactor. A silicon substrate on which a 3 nm thick Au film was deposited was positioned 10 cm away from the powder source. Argon gas was continuously supplied at a rate of 500 sccm during the synthesis. The temperature of the powder sources was set to 1000 °C. The substrate was approximately maintained at 800 °C to synthesize the NWs. The composition was controlled by changing the ratio of the ZnS and GaP powder. The structure and composition of the products were analyzed by scanning electron microscopy (SEM, Hitachi S4700), field-emission TEM (FEI TECNAI G2 200 kV), high voltage TEM (HVEM, Jeol JEM ARM 1300S, 1.25 MV), and energy-dispersive X-ray fluorescence spectroscopy (EDX). High resolution XRD patterns were obtained using the 9B beamlines of the Pohang Light Source (PLS) with monochromatic radiation. The spot size of X-ray spot (9B beamlines of the Pohang Light Source) is 2 cm diameter. The size of the nanowires sample (on the silicon substrate) is usually 1 cm2, which is smaller than the spot size. Therefore, the X-ray diffraction pattern represents the average information on the sample. The Raman spectra were acquired using the 514.5 nm line of an argon ion laser. UV−visible absorption spectra of the samples were recorded using a spectrometer (Cary 5000, Agilent Tech.). Photoluminescence (PL) measurements were carried out using a He−Cd laser (λ = 325 nm) or diode laser (λ = 375 nm) as the excitation source. A nanosecond pulsed Nd:YAG laser emitting at 266 nm (fourth harmonic) with a pulse energy of 20 mJ is used as the excitation source for the ZnS NWs. The time-resolved PL spectrum was measured using a timecorrelated single-photon counting (TCSPC) system. A pulsed diode-laser head (LDH−P-C-375, PicoQuant) coupled with a laser-diode driver (PDL 800-B, PicoQuant) was used as excitation source with a pulse width of less than 70 ps and a repetition rate of 20 MHz. The excitation wavelength was 375 nm. The fluorescence was spectrally resolved using a monochromator (SP-2150i, Acton) and its time-resolved signal was measured by a TCSPC module (PicoHarp, PicoQuant) with MCP-PMT (R3809U-59, Hamamatsu). The total instrument response function (IRF) was less than 130 ps, and the temporal resolution was less than 10 ps. The deconvolution of the decay curve, which separates the IRF and actual decay signal, was performed using fitting software (FluoFit, PicoQuant) to deduce the time constant associated with each exponential decay curve. The PL decays were usually analyzed by the multiexponential model given by exp(−t/τi). Photolithography was used to deposit the Ti (20 nm)/Au (80 nm) electrode structure onto a Si substrate with a 1 μm thick thermally grown SiO2 layer, by sputtering using a patterned mask. The gap distance between the electrodes was 1.5 μm. The NWs were aligned using the dielectrophoresis (DEP) force, which maintained their position after the solution dried out. A top metal electrode (Pt) was deposited on the contact position of the NW using a focused ion beam (FIB). We tested the devices on the probe station with parametric test equipment (Agilent E5270A) at room temperature. A diode laser (365 nm, 20−200 mW) was used as the light source.
for all measurements. The I−V curves were almost linear within the measured range (0−10 V). The photocurrents increased linearly with light intensity (1−10 mW). I−t curves at a bias voltage of 5 V were collected in real time under a series of on/ off cycles (Figure 6b). The current instantly increased ( 0.7) exists distinctively in the WZ phase. A unique core−shell structure was produced in the intermediate range (x = 0.4−0.7). This structure was composed of a ZB phase GaP-rich alloy (x = 0.4) core and a ZB-WZ phase ZnS-rich alloy (x = 0.7) shell with polytypic twinned superlattice structures along the ZB [111] (=WZ [0001]) growth direction. The growth direction is [111] (x = 0−0.7) and [011̅ 0] (x > 0.7). UV−visible absorption and PL spectra showed that the Eg (2.4−3.7 eV) increases nonlinearly with increasing x, showing a significant bowing phenomenon. The Eg of the GaP-rich NWs decreases relative to that of the GaP NWs by ∼0.1 eV upon the incorporation of x = 0.1. The ZnS-rich alloy NWs exhibit a dramatic increase from 2.4 to 3.6 eV with increasing x. More importantly, the evolution of the WZ phase leads to enhanced PL emission intensity and a blue shift (70 meV for x = 0.9) with an increase of excitation power. Time-resolved PL measurements revealed a strong dependence of the decay time on the emission energy. Based on the PL data, we suggested a type-II pseudobinary heterojunction between the GaP-like and ZnS-like layers for the WZ phase ZnS-rich alloy. Compared with unitary and other compositional alloy NWs, the ZnS-rich alloy NWs exhibited a higher photocurrent, which opens up a new strategy for boosting the performance of photodetectors. The unique optical properties of ZnS-rich alloy NWs might show promise for advanced optoelectronic nanodevices. 5917
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S7: EDX, PL, and photocurrent data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by KETEP (20124030200120). The HVEM measurements were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH.
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REFERENCES
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