Electric-Field-Dependent Photoconductivity in CdS Nanowires and

May 29, 2012 - The quasi-periodic oscillations above the band edge in nanowires and nanobelts have been attributed to a Franz−Keldesh effect. The ex...
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Electric-Field-Dependent Photoconductivity in CdS Nanowires and Nanobelts: Exciton Ionization, Franz−Keldysh, and Stark Effects Dehui Li,† Jun Zhang,† Qing Zhang,† and Qihua Xiong*,†,‡ †

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 ‡ Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 ABSTRACT: We report on the electric-field-dependent photoconductivity (PC) near the band-edge region of individual CdS nanowires and nanobelts. The quasi-periodic oscillations above the band edge in nanowires and nanobelts have been attributed to a Franz−Keldesh effect. The exciton peaks in PC spectra of the nanowires and thinner nanobelts show pronounced red-shifting due to the Stark effect as the electric field increases, while the exciton ionization is mainly facilitated by strong electron−longitudinal optical (LO) phonon coupling. However, the band-edge transition of thick nanobelts blue-shifts due to the field-enhanced exciton ionization, suggesting partial exciton ionization as the electron−LO phonon coupling is suppressed in the thicker belts. Large Stark shifts, up to 48 meV in the nanowire and 12 meV in the thinner nanobelts, have been achieved with a moderate electric field on the order of kV/cm, indicating a strong size and dimensionality implication due to confinement and surface depletion. KEYWORDS: Photoconductivity, CdS nanowires and nanobelts, Stark effect, Franz−Keldysh effect, Exciton ionization he electric field dependence of optical absorption (electroabsorption) near the optical band edge has been extensively investigated due to its promising applications in the electroabsorption modulators.1−3 When an electric field is applied in bulk semiconductors, the wave functions of electrons and holes overlap, leading to the exponential decay absorption tail below the bandgap exp[−σ(hν − E0)3/2], where hν is the photon energy, E0 is a fitting parameter, and σ is related to the material and the electric field, and quasi-periodic oscillations above the bandgap. This effect is known as Franz−Keldysh effect.4,5 When the exciton formation due to Columbic interaction is included, the absorption tail below the bandgap exhibits a simpler exponential dependence of exp[−σ(hν − E0)].3,6 Only in high quality direct-gap semiconductors can exciton peaks be identified in optical absorption when the thermal energy kBT is smaller than the exciton binding energy. Applying an electric field would lead to a red-shift of the exciton peak which is quite similar to the Stark effect in the hydrogen atoms and is thus also referred as Stark effect. In addition, the applied electric field may give rise to exciton dissociation,2,7,8 resulting in the exciton peaks in the PC spectrum. In low-dimensional systems, the Franz−Keldysh effect, Stark effect, and exciton ionization exhibit different behavior compared with their bulk counterparts. To handle the lowdimensional nanostructures such as quantum well structures, the quantum-confined Franz−Keldysh effect has been proposed in which the absorption at the band-edge region exhibits a small and closely spaced steplike behavior. The normal Franz−

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Keldysh effect is recovered when the confinement diminishes.9 The Stark effect is seldom observed in bulk semiconductors due to the small magnitude and severe peak broadening under an electric field. However, in quantum size regime, the magnitude of Stark effect increases dramatically due to the change of density of states (DOS) and the increasing of the exciton binding energy.10 The Stark effect in quantum size regime is usually called quantum-confined Stark effect to disinguish from its bulk counterpart in semiconductors.2,11 A requirement for the quantum-confined Stark effect to be observed is that the size of the nanosturctures in at least one dimension should be comparable to or smaller than their bulk exciton Bohr radius. As a result, the exciton binding energy increases significantly compared to bulk, suggesting that a larger electric field is required for exciton ionization. Nevertheless, the enhanced electron−LO phonon interaction at nanoscale may promote the rate of the exciton dissociation verified by the appearance of exciton contribution in PC spectrum.12 A large Stark shift (more than 2.5 times of bulk exciton binding energy) has been observed in quantum wells, at an electric field of 50 times larger than the ionization field which is defined as Eb/aB, where Eb is the bulk exciton binding energy and aB is the bulk exciton Bohr radius.11 The quantum-confined Stark effect has also been observed in nanocrystalline quantum dots by photoluminescence spectroscopy.13 Received: February 23, 2012 Revised: May 17, 2012

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Figure 1. Photoconductivity of CdS nanobelt/nanowire FET devices. (a) Schematic of the experimental setup. Inset to the left corner: an SEM image of a typical CdS nanobelt FET device. The scale bar is 1 μm. (b) Photocurrent spectra and (c) the gain of the nanowire and nanobelt devices. The applied source-drain electric field is 2 kV/cm.

two beams: one was used to illuminate the devices mounted on the cryostat cold finger while the other was directed onto a pryoelectric detector (Newport) to monitor the intensity of the incident light, in order to normalize the photon flux at different wavelengths. The photocurrent of a FET device and light intensity signals were both measured by two lock-in amplifiers (Stanford SR830) coupled with a mechanical chopper which was used to modulate the incident light and to provide the reference signals to the lock-in amplifiers. The light power intensity was measured around tens of μW/mm2, and the detection resolution is ∼1 pA. The nanowire diameter and nanobelt thickness were determined by atomic force microscopy (AFM) height measurement relative to the silicon substrates. The PC experimental setup is schematically shown in Figure 1a. The inset to Figure 1a is an SEM image of an individual CdS nanobelt device. The channel length is 1 μm. The monochromatic light illuminates perpendicularly onto the device, and the photocurrent is measured in a two-probe configuration as shown in Figure 1a. The photocurrent spectra of a nanowire (the diameter of the nanowire is 120 nm) and nanobelts with different thickness ranging from 50 to 230 nm are shown in Figure 1b. The applied source-drain electric field is defined as the ratio of the source-drain voltage to the channel length of the devices. Here the applied source-drain electric field is 2 kV/cm. A few broad oscillation peaks (labeled as asterisks) appearing on the nanowire and thin nanobelt (50 and 65 nm) devices can be attributed to the Fabry−Pérot modes formed due to the silicon oxide cavity on the substrate and the nanobelts themselves.17 No such oscillation is observed from the thick belt (230 nm) because its thickness is comparable to the penetration depth using the optical absorption coefficient of 5 μm−1.22 The apparent decreasing of photocurrent at the higher energy side is due to the low incident light intensity in UV regime for a quartz tungsten halogen lamp. To exclude the influence from the variation of the incident photon flux on the photocurrent, gain is a better physical quantity to be evaluated, which is defined as how many electrons are collected by electrodes for each incident photon, i.e., G = (Iph/e)/ϕph, where Iph is the photocurrent and φph is the photon flux. The gain of all devices is around several thousands, which is in an appropriate range as previously reported.14,16 The spectral response at the band-edge region exhibits a noticeable dimensionality and size dependence as shown in Figure 1c. A distinct sharp peak around 2.43 eV can be resolved from the nanowire device, while a small peak near 2.48 eV and

As compared with plentiful reports on electroabsorption in 2D quantum wells and 0D quantum dots, investigations on the electroabsorption of 1D nanowires and nanobelts are rarely reported. The challenge is the technical difficulty to measure optical absorption on individual nanowires or nanobelts. PC spectroscopy is an ideal approach to shed light on electroabsorption on an individual level. There has been considerable attention paid to the photodetection of semiconductor nanowires or nanobelts in the past decade;14−18 however, the electroabsorption of semiconductor nanowires or nanobelts as a dependence of electric field strength, size, and dimensionality remains elusive. Herein, we report a systematic investigation of PC spectroscopy on individual CdS nanowires and nanobelts at room temperature and low temperatures. The Franz−Keldysh effect has been identified by the exponential decay band tail and quasi-periodic oscillations above the band gap. Size and dimensionality dependence of Stark effect has been demonstrated at room temperature for the first time. We also find that exciton ionization is dominated by electron−LO phonon coupling in the nanowires and thin nanobelts, while the fieldinduced ionization plays a more significant role in the thick nanobelts, in which the electron−LO phonon coupling is weak. The CdS nanobelts were synthesized in a home-built vapor transport chemical vapor deposition (CVD) system. Detailed growth conditions, morphology, and crystalline characterizations have been published elsewhere.19 The CdS nanowires were grown using the same method but with different parameters.20 The width and the length of the nanobelts are several micrometers and tens of micrometers, respectively. The as-grown nanobelts and nanowires were sonicated into isopropanol solution and then dispersed on a 500 nm SiO2/ Si substrate. The Ti/Au (60/50 nm) electrodes were patterned by electron beam lithography followed by thermal evaporation and lift-off. Photoluminescence spectroscopy (PL) measurements of the individual CdS nanowires and nanobelts were carried out under a micro-Raman spectrometer (Horiba-JY T64000) in the backscattering configuration,21 excited by an Ar ion laser (457 nm) with a power density of 5 W/mm2. For the low-temperature measurements, a liquid helium continuous flow cryostat (Cryo Industry of America) was used to control the temperature from 4.2 to 300 K. The PC measurements were performed using a home-built PC setup. A quartz tungsten halogen lamp (250 W) was used as an excitation source, which was dispersed by a monochromator (Horiba JY HR320) with an energy resolution of 3 meV. The monochromatic light beam output was collimated and split into B

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Figure 2. PL (solid curves) and gain spectra (symbol curves) of (a) a 120 nm nanowire, (b) a 50 nm nanobelt, (c) a 65 nm belt, and (d) a 230 nm nanobelt at room temperature and low temperatures. The applied source-drain field is 2 kV/cm.

emitted photons causing the reabsorption of the emitted photons. As a result, the reabsorption effect must be taken into account which leads to the red-shift of the luminescence spectrum.30 From a detailed thickness-dependent PL spectroscopy investigation, the emission peak position without reabsorption can be deduced by the extrapolation of the PL peak position versus thickness, which exactly coincides with the broad band-edge transition position of the PC spectrum.24 Therefore, the broad band-edge transition corresponds to the mixture of FXA and FXB. As the temperature increases, the increase of the red-shift due to the reabsorption leads to the larger difference between the emission peaks and the broad band-edge transition as seen from the Figure 2d. Once the positions of FXA and FXB are confirmed, the position of FXC can be assigned correspondingly according to the energy difference for different temperatures as shown in Figure 2a−d. The agreement of exciton peaks in PL and PC spectra shows a negligible Stokes shift, which proves the crystalline quality of our samples,14 consistent with recent reports of n-type CdSe nanowires studied by our group.31,32 It should be noted that the PC spectrum depends on both carrier generation and carrier transport to electrodes. Therefore, PC is related but not exactly equivalent to the optical absorption spectrum. However, PC measurement can be used to study the electroabsorption if the transport of photogenerated carriers is efficient regardless of the excited photon energy. For the incident photon energy above the bandgap, free carriers are generated which can be immediately swept out of the devices. However, absorption at the exciton peaks will initially create excitons bound together via Coulombic interactions which will not contribute to the photocurrent unless they are ionized through interactions either with optical phonons33 or by field ionization34 (including both the internal the electric field due to the surface depletion and the external electric field exerted through electrodes) in a time scale smaller than their lifetime.11 In our CdS samples, the electron−phonon coupling is significant, which has been confirmed by the resonant Raman scattering experiments in similar II−VI nanostructures.35 The strong electron−phonon coupling would dissociate the excitons leading to an increased photocurrent at the exciton peak positions. Besides, the strong surface electric field in low-dimensional nanomaterials (nanowire or nanobelt) and applied source-drain electric field can also cause the dissociation of the excitons. The exciton ionization has been observed in CdS crystal, where the correlation measurements between exciton absorption and PC have been conducted. Those two spectra gave the same

another rather broad bump close to 2.56 eV are identified for those thinner nanobelts (50 and 60 nm). For the thick nanobelt (230 nm), only a broad band-edge transition is resolved at ∼2.45 eV, which exhibits a very long tail at the lower energy side. The long tail has various possible origins including phonon-assisted transition, excitonic effect, impurities, and Franz−Keldysh effect.6,23 Away from the band edge at the higher energy side, oscillations are observed for all devices but more pronounced for the nanowire and thicker nanobelt. The band edge exhibits a gradual red-shift as the thickness of the nanobelts increases, which has also been observed in PL spectra and has been presented elsewhere.24 In order to assign those peaks and bumps nearby the band edge unambiguously, the PL spectra have been taken at room temperature and low temperatures as shown in the comparisons of PL and PC spectra (Figure 2). Because of the crystal field and spin−orbit interaction, the valence band splits into three bands. The corresponding excitons are labeled as free exciton A (FXA), B (FXB), and C (FXC) with characteristic energies of 2.550, 2.568, and 2.629 eV, respectively.19 In the nanowire, the emission peak energy is higher than the sharp peak of the PC spectrum with a blue-shift of 45 meV, larger than the exciton binding energy of 28 meV.25,26 Therefore, the sharp peak of the PC spectrum can be tentatively attributed to FXB, while the PL emission is due to the free carrier recombination. The small bump ∼14 meV below the sharp FXB peak can be assigned as FXA as labeled in Figure 2a. The considerably large energy difference (∼59 meV) between FXA and PL emission is due to a Stark effect in nanowire, which introduces a strong red-shift in the PC spectrum as will be explained in detail later. We can assign the peak in the PC spectrum from 65 nm nanobelt as FXB and the small transition just below FXB as FXA (as indicated in the Figure 2c) by comparison of the PL and PC spectra at 77 K. Similarly, FXA and FXB for the 50 nm belt are assigned at 180 K as labeled in Figure 2b. The 50 nm belt device is broken down below 180 K so we do not show the PC spectrum at 77 K. Based on the temperature-dependent PL and PC spectra, the peaks and bumps at room temperature are properly and unambiguously assigned. It is not surprising that exciton peaks can be identified in PC spectrum at room temperature, as it has been demonstrated that the FXA emission is dominated in CdS nanobelts at room temperature,25,27 which is similar as in ZnS nanowires.28 For the 230 nm nanobelt, the thickness is larger than the half of the wavelength of emitted photons inside the belt, which is around 97 nm.29 The internal reflection can occur at the surfaces or interfaces of the nanobelts which will trap the C

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Figure 3. (a−d) Normalized gain spectra (red curves) and their corresponding second derivative (blue curves) for the nanowire and nanobelts with an applied source-drain electric field of 2 kV/cm near the band-edge region at the higher energy side. The numbers labeled in each figure indicate the index of the oscillations. Spectra are offset vertically for clarity. (e) Extrema of the oscillations versus the index of the oscillations (solid dots). The experimental data are well fitted by the function En ∝ n2/3 shown as solid lines.

Figure 4. Electric field dependence of the normalized gain (a−d) and their corresponding second derivative (e−h) of nanowire and the nanobelts with various sizes near the band-edge region: (a, e) 120 nm nanowire, (b, f) 50 nm nanobelt, (c, g) 65 nm nanobelt, and (d, h) 230 nm nanobelt. The red arrows in (a) and thin lines in (b, c, d) indicate the positions of excitons of interest. All spectra are offset vertically for clarity except (e).

has been observed in PC of GaN nanowire, which was attributed to the Franz−Keldysh effect induced by surface electric field.6 Nonetheless, a few other underlying processes may also give rise to the exponential tail, such as electron− phonon interaction,23 structural disorders,38 and impurities.39 Therefore, both exponential tail and the oscillation extrema scaling must be taken into account to pin down the possible Franz−Keldysh effect. In addition, the decay band tail of all PC spectra can be well fitted by an exponential function, which is another evidence of the Franz−Keldysh effect. We further zoom-in the normalized gain of PC spectra (Figure 1c) of the nanowire and nanobelts near the band-edge region at the higher energy side as shown in Figure 3a−d. The second derivatives of gain versus energy for the nanowire and nanobelts are given in Figure 3a−d as blue dotted lines in order to extract the extrema of the oscillations versus the oscillation index. The quasi-periodic oscillations have been unambiguously identified in all spectra. The extrema energies En can be well fitted by the function En ∝ n2/3 shown in Figure 3e. Therefore, the quasi-periodic oscillations at the higher energy side of the

spectral configuration, which manifests that excitons are ionized and contribute to the photocurrent.33 Therefore, the presence of the exciton peaks in our PC spectra demonstrates the exciton dissociation in our experiments. An electric field applied to a semiconductor would cause the wave functions of electrons in conduction band and holes in valence band to change from Bloch functions to Airy functions.3 This transition makes both wave functions leak into the bandgap, further leading to the nonzero overlap integral between them,1 which is demonstrated by the nonzero absorption below the band gap. This phenomenon is called the Franz−Keldysh effect.4,5 The exponential decay tail below the bandgap and the quasi-periodical oscillations above the band gap are two characteristic fingerprints of the Franz−Keldysh effect. At the higher energy side near the band gap, the oscillations show quasi-periodic characteristics. The extrema of the oscillations (En) as a function of the oscillation index (n) can be expressed as En ∝ n2/3.36,37 Far away from the band edge at the higher energy side, the oscillations average out because the Airy functions are not strictly periodic.1 The exponential tail D

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band edge can be attributed to the Franz−Keldysh effect. The oscillations in the nanowire and thicker nanobelt devices are more pronounced than those in thinner belt devices. This is probably due to the transition from a bulk Franz−Keldysh effect to a quantum-confined Franz−Keldysh effect, as proposed in 2D quantum well structures.1 The electric field dependence of PC gain spectra and their corresponding second derivative near the band edge are shown in Figure 4. The spectra have been normalized by the maximum value of each spectrum and offset vertically in order to compare the field dependence except (e). Based on the previous assignments, FXB in the nanowire and 65 nm nanobelt, FXC in the 50 nm nanobelt, and the broad band-edge transition in the 230 nm nanobelt are selected to examine the trends versus field strength. Because of the strong surface electric field broadening2 and small energy difference of 17 meV between FXA and FXB, the FXA and FXB are not well resolved at room temperature. Therefore, we select FXC for 50 nm belt. For the nanowire and thinner nanobelt devices, FXB and FXC peaks exhibit a gradual red-shift and a considerable broadening as the electric field is increased. At certain electric field strength, the broadening is so significant such that those exciton peaks in PC can no longer be resolved; for instance, 4 kV/cm for the nanowire and 65 nm nanobelt and 6 kV/cm for the 50 nm nanobelt. Strikingly, the broad band-edge transition (d, h) in PC for a 230 nm nanobelt exhibits a significant blue-shift, and the band tail of the PC spectra becomes steeper and steeper as the electric field is increased. The possible heating effect due to incident photons, which usually gives rise to a red-shift of band-edge emission in semiconductors, can be ruled out since the power density of the incident collimated photons in our PC experiments is very small, around tens of μW/mm2. The heating effect due to photocurrent itself can also be ruled out. To confirm this, the PL spectra and the photocurrent spectra have been taken simultaneously excited by a 457 nm laser (Ar ion laser) with a power density of 5 W/mm2, several orders higher than the power density of the broad band light source used in our PC experiments. Therefore, the photocurrent is expected to be much higher than that in PC spectroscopy. During a continuous 2 h running, the PL spectra do not show any noticeable red-shift, suggesting no Joule heating effect due to photocurrent itself. From the second derivative of the gain spectra, the peak positions of interest can be readily extracted. A plot of the FXB peak position as a function of the applied electric field in the nanowire is shown in Figure 5. The plot can be well fitted by the sum of a linear and quadratic functions of the field13 E(ξ) = E(0) + μξ +

1 2 αξ 2

Figure 5. Field dependence of the FXB position in the nanowire and in the 65 nm nanobelt, the FXC in the 50 nm nanobelt (right axis), and the band-edge transition in the 230 nm nanobelt. The curves are the fittings as described in the text.

spectrum (2.466 eV, Figure 2a) by 23 meV, consistent with the exciton binding energy of 28 meV within the experimental uncertainty, which verifies the aforementioned assignment. In the thinner nanobelts, peak positions versus the applied field can be well fitted by a linear function alone (Figure 5), indicating that only the dipole component plays a major role, which illustrates that the localized surface trap states mainly contribute to the Stark effect and the contribution from delocalized bulk exciton states is negligible, consistent with the large surface-to-volume ratio in those thinner nanobelts. The thickness of the nanobelts (50 and 65 nm, respectively) is less than the diameter of the nanowire of 120 nm. Therefore, it is expected that only surface trap states play a major role in the nanobelts, while both surface trap states and delocalized bulk exciton states contribute to Stark effect in the nanowire. Up to a 48 meV Stark shift was observed in the nanowire which is 2 times larger than the exciton binding energy. This contradicts to the bulk case where the Stark shift can only shifts ∼10% of the exciton binding energy before the exciton peaks become unresolvable caused by broadening.9 The Stark shifts are 12 and 6 meV for 50 and 65 nm nanobelts, respectively, both of which are also larger than 10% of the exciton binding energy. Those shifts are larger than the values observed in CdS single crystal, and the electric field applied here is also 1 order of magnitude smaller.42 The reason for the observation of the large Stark shifts under such a small applied electric field is not clear yet. Furthermore, there are some clues that the large Stark shifts might be due to the confinement induced by the surface depletion.24 For instance, the Stark shift in 50 nm nanobelt is larger than that observed in 65 nm nanobelt, and no Stark shift was observed in 230 nm thick nanobelt (Figure 5), which implies that the size effect indeed plays a significant role. Regarding the nanowire, one more dimensional confinement may lead to a much larger shift. However, the size of our samples (both for nanowire and nanobelts) is much larger than the bulk exciton Bohr radius required for the presence of the quantum confined Stark effect2 unless the surface depletion poses a very important influence to the confinement.24 However, it is required the electric field should be along with the confinement orientations for the quantum confined Stark effect to occur. In our case, the applied source-drain electric field between the top metal contacts has the perpendicular component, which probably contributes to the large Stark shift.43 With respect to the small applied electric field, possible

(1)

where E(ξ) is the peak position with an applied electric field ξ, E(0) is the peak position without the electric field, ξ is the field applied, and μ and α are the projections of the excited-state dipole and polarizability along the applied field, respectively. The presence of the linear and quadratic terms in the fitting function indicates that both polar character originated from the localized surface trap states40 and polarizable character due to the delocalized bulk exciton states41 contribute to the Stark effect in the nanowire. The fitting result of the coefficient of the quadratic term is quite small, meaning that the localized surface state contribution dominates. The zero-field peak position extrapolated (E(0) = 2.443 eV) differs from the peaks of PL E

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blue-shifts with the increase of the applied field as shown in Figure 6b, a strong evidence of the field-enhanced exciton ionization. The completely opposite shifts here compared to Figure 4c indicate that field induced exciton ionization dominates over Stark effect. At room temperature, the exciton ionization is dominated by electron−LO phonon coupling, and thus the electric-field-induced ionization is negligible in those thin belts and nanowires. In conclusion, the source-drain electric field dependence of the electroabsorption near the band-edge region of individual CdS nanowires and nanobelts has been investigated. The Franz−Keldesh effect has been demonstrated by the quasioscillations above the band edge of PC spectra. As the applied field increases, a pronounced red-shift and broadening of exciton peaks are identified in the nanowires and thinner nanobelts. The exciton peak position as a function of the electric field can be well fitted by the sum of a linear and a quadratic function of the electric field in the nanowire, indicating that both delocalized exciton states and localized surface trap states contribute to the red-shift. In comparison, surface trap states alone play a major role to the red-shift of the exciton peaks in the thinner nanobelts supported by the linear field dependence of exciton peak positions in PC spectra. In the thicker nanobelt, the band-edge transition shifts toward the higher energy side with the increase of the applied electric field, which might be due to the field-enhanced exciton ionization. From the blue-shift of the exciton peaks with the applied electric field at 77 K when the electron−LO phonon coupling is considerably weaker, we deduce that the exciton ionization at room temperature is dominated by the strong electron−LO phonon coupling. The strong electric-field-dependent band edge in PC spectra can find promising applications in the electroabsorption modulators.

reasons include the surface electric field15 and nonuniform distribution of the applied electric field.44 With respect to the blue-shift of the broad band-edge transition in the 230 nm nanobelt with the increasing of the applied electric field, the energy of transition versus applied electric field can also be fitted by an exponential function E(ξ) = A exp[−(Em − eξaB)/B)] + E(0), where A and B are constants, Em is exciton binding energy, m is the exciton energy level index, aB is the bulk exciton Bohr radius, ξ is the applied field, and E(0) is the photon energy without field applied. This blue-shift trend might be explained by field enhanced exciton ionization.34 Between the exciton ground level (m = 1) and conduction band, a great amount of high-lying exciton levels (m > 1) exist. The electrons resonantly excited from the valence band to those high-lying exciton levels (m > 1) should have much larger possibilities to be ionized by tunnelling due to both the smaller exciton binding energy (Em = E1/m2) and the reduction of barrier height (Em − eξaB) induced by the electric field ξ.7 Hence, the larger the applied electric field is, the larger the tunneling possibility (exp[−(Em − eξaB)/B]) is, which reflects the probability that the excitons in those high-lying levels (m > 1) can be ionized before decay to the ground level (hence no contribution to photocurrent). It should be noted that the applied electric field here is much smaller than the ionization field for CdS, which is on the order of 105 V/cm.42 Therefore, the exciton ionization induced by the electric field is mainly through the tunnelling rather than direct ionization. The ionization for those high-lying excitons gradually changes from tunnelling to direct ionization as the applied field increases further. As a result, the band-edge transition shows a blue-shift. In addition, the band tail of the PC spectra becomes steeper and steeper as the increase of the electric field, which also indicates the larger probabilities for those high-lying excitons to be ionized at higher fields. In order to further understand how Stark effect and exciton ionization compete in the nanobelts, PC measurements have been conducted at 77 K on the 65 nm nanobelt. Electron−LO phonon coupling has been shown to be considerably weak below 100 K in CdS.45 Therefore, the exciton ionization is presumably not very significant at 77 K. As shown in Figure 6a,

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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 work is mainly supported by Singapore National Research Foundation through a NRF fellowship grant (NRF-RF200906). Q.X. also acknowledges the strong support by start-up grant support (M58113004) and New Iniative Fund (M58110100) from Nanyang Technological University (NTU).

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REFERENCES

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Figure 6. (a) PL (red curve) and PC spectra (blue curve) of the 65 nm thick nanobelt at 77 K. (b) Electric field dependence of normalized photocurrent spectra at 77 K. The vertical lines indicate the positions of FXB and FXC. The spectra are offset vertically for clarity.

the peak assignment in PC spectrum can be supported by PL spectrum taken from the same belt at 77 K. Accordingly, FXA, FXB, and FXC are assigned in PC spectrum as labeled in Figure 6a. The broad emission band at the lower energy side is due to donor−acceptor pair recombination formed between sulfur and cadmium vacancies.19 Both FXB and FXC show noticeable F

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dx.doi.org/10.1021/nl300749z | Nano Lett. XXXX, XXX, XXX−XXX