Photoconduction Properties in Single-Crystalline Titanium Dioxide

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Photoconduction Properties in Single-Crystalline Titanium Dioxide Nanorods with Ultrahigh Normalized Gain R. S. Chen,*,† C. A. Chen,‡ H. Y. Tsai,§ W. C. Wang,‡ and Y. S. Huang‡,§ †

Graduate Institute of Applied Science and Technology, ‡Department of Electronic Engineering, and §Graduate Institute of Electro-Optical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan, R. O. C. ABSTRACT: We report on a systematic photoconductivity (PC) study on the individual titanium dioxide (TiO2) nanorods (NRs) with single-crystalline quality. The photoconductive gains (device-oriented parameter) and their corresponding normalized gains (material-oriented parameter) of the TiO2 NRs have been defined and compared. The quantitative results show that the indirect-bandgap TiO2 NR exhibits a competeable photoconduction efficiency with the maximal (saturation) normalized gain (Γn) at 2 × 10−4 m2 V−1 in comparison to the direct-bandgap zinc oxide (ZnO) nanowires (Γn ∼ 1.2 × 10−4 m2 V−1). In addition, the maximal normalized gain is also over 4 orders of magnitude higher than that reported for the polycrystalline nanotube counterpart. The photoconduction mechanism is also proposed for the highly efficient photoconduction properties in this titania nanostructure. The results demonstrate the potential of an ultrasensitive ultraviolet photodetector using TiO2 NRs and the superior efficiency for charge transport in boundary-free titania one-dimensional nanostructures, which is crucial for dye-sensitized solar cell and photochemical device applications.

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INTRODUCTION Titanium dioxide (TiO2) one-dimensional (1D) nanostructures, such as nanotubes (NTs), nanowires (NWs), and nanorods (NRs), have been widely used as the photoelectrode materials in dye-sensitized solar cell (DSSC),1−8 photocatalysis,9−11 water splitting,12−14 and sensor15,16 applications. In addition to the large surface area, 1D nanostructure provides a continuous transport path for carriers, which is essential for optimizing the energy-harvesting efficiency. Among them, polycrystalline and highly porous structures have been frequently used to gain more surface area, but at the same time highly dense grain boundaries and defective texture could significantly lower the charge transport efficiency and material stability.1−4,9,10,12,13,15 Nevertheless, the single-crystalline nanomaterials would be the potential solution for such problems of polycrystalline and nanoporous materials owing to their longrange ordered and energetically stable structures. The excellent performances on the applications of photochemical and photoelectrochemical devices have also been demonstrated utilizing the monocrystalline 1D nanostructures of TiO2.5−8,11,14,16 Furthermore, a detailed fundamental study on the photoelectrical properties of the boundary-free TiO2 1D nanostructure remains a critical issue and has rarely been reported.17 Different from the measurements on the ensembles of nanostructures, directly probing the individuals would avoid the interface effect and could be able to define the material properties quantitatively. Fabrega et al. have reported the photoconduction properties in individual TiO2 NTs with polycrystalline quality.18 In their study, charge carriers were found to suffer from severe scattering during the transport, © 2012 American Chemical Society

which consequently led to the poor photocarrier collection efficiency. Here, we report on the photoconductivity (PC) study of the single-crystalline TiO2 NRs. The PC performance of the TiO2 NRs is quantified and compared by defining the photoconductive gain (Γ) and normalized gain (Γn). An ultrahigh Γn at saturation is obtained, which is over 4 orders of magnitude higher than the polycrystalline NTs18 and is comparable with the ZnO NWs19 in the literature. The potential mechanism underneath the highly efficient PC in this titania 1D nanostructure is also discussed.

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EXPERIMENTAL METHODS The TiO2 NRs used for this study were prepared by cold-wall metal−organic chemical vapor deposition (MOCVD) using the sapphire (100) substrate at the deposition temperature of 550 °C and oxygen pressure of 1.5 mbar. The details of the NR growth can be found in our earlier publications.20,21 The morphological and structural properties of the as-grown NRs were characterized using field-emission scanning electron microscopy (FESEM), X-ray diffractometry (XRD), Raman scattering spectroscopy, transmission electron microscopy (TEM), and selected-area electron diffractometry (SAD). The two-terminal single NR devices were fabricated using focusedion beam (FIB) deposition and platinum (Pt) as the contact metal. Individual NRs were dispersed on the insulating Si3N4(200 nm)/n-Si template with prepatterned Ti/Au microReceived: October 18, 2011 Revised: January 11, 2012 Published: January 12, 2012 4267

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electrodes prior to the FIB deposition. Electrical measurements were carried out on an ultralow current leakage cryogenic probe station (LakeShore Cryotronics TTP4) at room temperature. A semiconductor characterization system (Keithley 4200-SCS) was utilized to source the DC bias and measure the current. The variable monochromic light beam for PC spectrum measurements was provided by a PTI 101 monochromator installed with a 150 W xenon lamp. The He−Cd laser with 325 nm wavelength was used as the excitation source for the environment and power-dependent PC measurements. The incident power of laser was measured by a calibrated power meter (Ophir Nova II) with a silicon photodiode head (Ophir PD300-UV).

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RESULTS AND DISCUSSION Figures 1a and 1b illustrate the FESEM micrographs of the well-aligned TiO2 NRs grown on the sapphire (100) substrate. The predominant rutile phase and ⟨001⟩ preferred orientation along the long axis of the TiO2 NR ensemble are verified by an XRD pattern (Figure 1c). The Raman spectrum also confirms the single rutile phase of the as-grown TiO2 NRs (Figure 1d); therein two major peaks centered at 441 and 610 cm−1 are, respectively, assigned to the Eg and A1g modes of rutile TiO2. The Raman signals of the TiO2 NRs show a slight red-shift in peak position and broader line width (fwhm = 37/48 cm−1 for the Eg/A1g mode) while compared to bulk crystals,22 which is attributed to both phonon confinement and residual stress effects.23 The ⟨001⟩ long-axis orientation and the monocrystalline rutile structure of the individual TiO2 NRs are further confirmed by the TEM image and its SAD pattern as shown in Figure 1e. Figure 1f illustrates the clear lattice image of the TiO2 NR observed by the high-resolution TEM. Figure 1g illustrates a typical micrograph of the single-NR device with two Pt contact electrodes fabricated using FIB deposition. The dark current versus bias (id−V) curve measured in air ambience for the TiO2 NR device with diameter (d) of 300 nm is depicted in Figure 1h. The linear behavior indicates good ohmic contact of the single-NR devices. The dark conductivity (σ) estimated from the id−V measurement of the NR is around 1.9 ± 0.2 Ω−1 cm−1. Although the σ could slightly vary while measured on different TiO2 NR devices, the statistically estimated values locate in the range of 1−10 Ω−1 cm−1.24 The σ values have also been cross-checked using a fourpoint probe approach, finding low contact resistance in these photoconductor-type devices. The photocurrent (ip) response versus different excitation energy (E), ranging from 2.0 to 3.5 eV, has been measured at the DC bias (V) of 0.5 V in air ambience for the TiO2 NR, as depicted in Figure 2a. The ip value is defined as the measured current under the ultraviolet (UV) illumination subtracting the background dark current. The PC spectrum reveals an absorption shoulder at ∼2.67 eV before a sharp ip increase at around 2.94 eV. The origin of the absorption shoulder could be attributed to the bound exciton emission resulting from TiO6 octahedra near defects in rutile TiO2.25,26 The major photoresponse close to the 3.0 eV position is consistent with the bandgap energy of the TiO2 single crystal with rutile structure.27 The ip responses of a single TiO2 NR under a single longpulse above-bandgap excitation at the wavelength of 325 nm and under the bias of 0.1 V in vacuum, air, and pure oxygen (760 Torr) ambiences are shown in Figure 2b. A constant background current has been subtracted for the three original

Figure 1. (a) Top-viewed and (b) cross-sectional-viewed FESEM images, (c) the XRD pattern, and (d) the Raman spectrum of the wellaligned TiO2 NR ensemble grown on the sapphire (100) substrate by MOCVD. Scale bar = 400 nm for (a) and (b). (e) The TEM image and its SAD pattern and (f) the high-resolution TEM image focused on an individual TiO2 NR. Scale bars = 200 and 2 nm for (e) and (f), respectively. (g) A typical micrograph of the single TiO2 NR device fabricated by the FIB approach. Scale bar = 2 μm. (h) The dark current versus applied bias measurement in air ambience for the single TiO2 NR with diameter of 300 nm.

curves to reveal the ip values. Under the 1 ks UV exposure, the ip in vacuum reaches to a higher level and takes a much longer time to reach steady state than those measured in air and oxygen ambiences. The ip at 1.0 ks in vacuum is roughly 1.6 times higher than the values obtained in air and oxygen. The ratio of ip value measured in vacuum to that in air is found to increase persistently while UV exposure time increases. In addition to the environment dependence, the ip responses under different incident light intensity (I) are also illustrated in Figure 3a. The figure shows that the ip level gradually increases, while the I increases for over 5 orders of magnitude. It is noticed that the ip rise time significantly shortens with increasing power. The estimated ip values as a function of I reveal a near linear dependence (Figure 3b), at the lowest power density region (I = 2−8 × 10−3 W m−2). While I ≥ 1.6 × 4268

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Figure 2. (a) Photocurrent spectrum measured in the excitation energy range from 2.0 to 3.5 eV at the bias of 0.5 V in air ambience for the single TiO2 NR. (b) The photocurrent response under a single long pulse excitation in vacuum, air, and pure oxygen ambiences at the excitation energy of 3.82 eV and the bias of 0.1 V for the single TiO2 NR with diameter of 300 nm. A constant background current of air has been subtracted, and the dark current difference between the different ambiences could also be observed.

10−2 W m−2, the ip becomes less sensitive to the excitation power and gradually saturates to a constant level. To investigate the environment and power-dependent PCs and their underneath mechanism, the Γ, a parameter that decides the photocarrier collection efficiency in a photoconductor, is evaluated. As photoconduction is a two-step process including optical absorption (determined by the net quantum efficiency (η)) and photocarrier transport (determined by Γ), the ip is linearly proportional to η and Γ following the equation28−30 ip =

q P ηΓ E

Figure 3. (a) Photocurrent response curves under different incident light intensity and (b) the photocurrent and (c) the calculated gain and normalized gain values as a function of incident light intensity ranging from 2 × 10−3 to 8 × 102 W/m2 for the single TiO2 NR with diameter of 300 nm at the excitation energy of 3.82 eV and the bias of 0.1 V. The linear relationship between the photocurrent and intensity in the lowest intensity region is observed by the linear plot shown in the inset of (b). The levels of normalized gain estimated from the ZnO NWs19 (pink dash) and the polycrystalline TiO2 NTs18 (cyan dash) are also marked for comparison in (c).

(1)

where q is the unit electron charge; P is the incident optical power on the projected area (A = dl) of the measured NR and can be calculated as P = Idl; and l is the interdistance between two contact electrodes of the single-rod devices. Considering the excitation energy E = 3.82 eV and assuming η = 100%, the Γ values of the TiO2 NRs are estimated under different I ranging from 2 × 10−3 to 8 × 102 W m−2, as shown in Figure 3c. From the plot, the Γ reveals sensitive power dependence and follows an inverse power-law in the intensity range of 3.2 × 10−2 to 8 × 102 W m−2. Once I goes below a critical intensity (Ic) at ∼8 × 10−3 W m−2, the Γ value gradually reaches a constant with a maximum near 1.3 × 106. The value is nearly 6 orders of magnitudes higher than that reported for polycrystalline TiO2 NTs (Γ < 3).18 In comparison to the other metal oxide nanomaterials, the result is still higher than those of the SnO2 (Γ = 8 × 103)31 and V2O5 NWs (Γ ∼ 1.3 × 103)32 but about 2 orders of magnitude lower than the highest reported data for ZnO NWs (Γ ∼ 1 × 108).19 As Γ conceptually stands for the circulating number of carriers transported through the photoconductor per unit time before the recombination, the Γ is also defined as the ratio of

carrier lifetime (τ) to transit time (τt) between two electrodes and can be expressed as29,30,33 Γ=

τ V = 2 τμ τt l

(2)

where μ is the mobility. According to eq 2, Γ is a function of the experimental parameters (V and l) and the material characteristics (τ and μ). Comparison of material properties between different photoconductors should be made under the same experimental conditions. Accordingly, a parameter, named Γn, which is defined as Γn = ητμ 4269

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quantitative result is consistent with the previous understandings, in which single-crystalline TiO2 reveals relatively low mobility35−37 and optical absorption efficiency in comparison to ZnO. The long-lifetime carrier transport in metal oxide semiconductors with intrinsically n-type conducting nature, such as ZnO, SnO2, TiO2, etc., is frequently attributed to the oxygen sensitization mechanism.19,31,38,39 According to the description of a conventional model, photoconduction is a four-step process: (i) In the dark and oxygen ambience, adsorption of foreign oxygen molecules creates electron trapping sites on the surface. After an electron is captured by the surface trapping state, the oxygen molecules turn into negative ions [O2(g) + e− → O2−(ad)] that enhance the upward bending of the energy band at the surface. (ii) Under photoexcitation, electron−hole pairs are generated [hυ → e− + h+] and (iii) subsequently separated by the surface band bending. (iv) Excess holes would be pushed to the surface following the built-in field imposed by the surface band bending and recombine with negative charged adsorbed oxygen [h+ + O2−(ad) → O2(g)]. Accordingly, due to the deficiency of holes, the recombination of the unpaired electron would be improbable in the neutral core of NRs, resulting in a longer lifetime. Recombination could only take place while oxygen molecules could readsorb on the surface following step (i). The cycle of the PC mechanism is schematically shown in Figure 4. Following the idea, the

is adopted to express the photoconductor performance objectively.18,34 Normalizing the Γ value via excluding the contributions of V and l and taking the η into account, the Γn actually can stand for the combined efficiency of the two-step PC process, including optical-to-electrical energy conversion (i.e., η) and photocarrier transport efficiencies (i.e., τμ), in a photoconductor. Although the η can be estimated from the absorption coefficient and reflectivity under the assumption of normal light incidence on a planar absorbing surface,28 the realistic cases in the 1D nanostructures are much more complicated. Due to the uncertainties of the cross-sectional shape, surface roughness, and angle of surface sidewall to the incident light, the estimated η value could deviate a lot from the assumed condition. Therefore, comparing the combined efficiencies of PCs between different nanomaterials could avoid the uncertainty and reflect the real surface conditions. Combining eqs 1, 2, and 3, Γn can be simply calculated from the measured ip, which is rewritten as ip =

q V qV P η 2 τμ = P Γn E l E l2

⇒Γn =

E l2 ip qV P

(4)

(5)

The calculated Γn versus I is also illustrated in Figure 3c under the experimental conditions of E = 3.82 eV and V = 0.1 V. The maximal (saturation) Γn at the lowest power density of 2 × 10−3 W m−2 for the TiO2 NR with d of 300 nm and l of 4.0 μm can reach to 2 × 10−4 m2 V−1. Although the Γn is highly dependent on the excitation power, its saturation position can represent a maximal value or the best photoconduction efficiency for the light detection at the low intensity. The result shows the monocrystalline TiO2 NRs with the saturation Γn which is over 4 orders of magnitude higher than the polycrystalline TiO2 NTs (Γn ∼ 10−8 m2 V−1).18 Although the lowest detection power density can only reach ∼50 W m−2 for the TiO2 NTs, the linear relationship of ip and photon flux still allow us to assume the τ or Γn, still keeping at a constant without power dependence following the conventional hole trapping mechanism. According to Fabrega et al.,18 the low mobility due to the polycrystalline structure of the NTs could be the reason for its low Γn and also lead to the huge difference in the PC performances between the TiO2 NR and NT. While compared to the best recorded data in metal oxide counterparts, the maximal Γn is slightly higher than the maximal (saturation) value at 1.2 ± 0.4 × 10−4 m2 V−1 estimated from the high-gain ZnO NWs.19 Even if the direct-bandgap ZnO NW has been considered as the highly efficient photoconductor among the oxide semiconductor systems, we found that the TiO2 NRs with indirect gap can exhibit comparable PC performance, hence it is worth investigating the underneath mechanism. Although a conclusive explanation has not been achieved yet, the probable reasons are inferred as follows. According to eq 3, Γn is decided by the product of η, τ, and μ. The values of τ can be obtained via the time-resolved PC measurement. Statistically measured, the τ values for the individual TiO2 NRs are in the range of 80−400 s under the power density of 4−8 × 10−2 W m−2. The mean value at 240 s is over seven times higher than that (τ = 33 s) of the ZnO NWs.19 Accordingly, we may derive the ημ values at 3.8 ± 2.8 × 10−7 m2 V−1 s−1 of the TiO2 NRs, which are 11 ± 8% of the value (3.6 × 10−6 m2 V−1 s−1) of the ZnO NWs. The

Figure 4. Schematic oxygen-sensitized photoconduction mechanism in the TiO2 NRs. The electron−hole spatial separation induced by surface band bending is also shown, as depicted by step (iii).

recombination rate and τ of the excess electron are governed by the oxygen adsorption rate, which is expectedly much lower in oxygen-deficient ambience. The hypothesis can be supported by the observation of enhanced ip and prolonged τ in vacuum compared to those in air ambience. The role of an adsorbed oxygen molecule acting as a surface acceptor state could also be confirmed by the lower background current (i.e., id) measured in an oxygen-containing environment than that in vacuum. These arguments agree with the observation of environmentdependent measurement for the TiO2 NRs in Figure 2b. However, the aforementioned model might be oversimplified and unable to explain the power-dependent Γn in Figure 3c. While τ is determined by the oxygen adsorption rate that depends on the ambience, we may expect the value is a constant as the saturation behavior of Γn at low I region. Once the intensity is higher than a trap-filling intensity, i.e., Ic, the Γn decreases rapidly with increasing I following a power-law of Γn ∝ I−0.84. Usually, the change in the power-dependent behavior 4270

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2112-M-011-001-MY3 and NSC 99-2738-M-011-001) and the National Taiwan University of Science and Technology.

is simply attributed to the onset of carrier bimolecular recombination at higher excitation intensity due to the hole trap saturation.19,40,41 Nevertheless, the untrapped electrons could still enjoy a long lifetime due to the spatial separation induced by surface band bending. The statement actually implies that the high Γn of TiO2 NRs originated from not only the oxygen sensitization phenomenon but also the spatial separation mechanisms at the I higher than Ic. In addition, the sharp decrease of Γn is also consistent with the intensity effect on the space charge region. While τ of the carrier is controlled by the surface band bending, creation of the electron−hole pair by illumination will induce a photovoltaic effect and thus reduces the surface band bending and shortens the τ. According to the literature,28,42,43 the Γ (or Γn) can be very sensitive to the I with the relationship of inverse powerlaw, Γ ∝ I−κ. Here, κ is in the range of 0.8−0.9, which is higher than the value (κ = 0.5) induced by the intrinsic recombination mechanism in the bulk.40,41 The κ value at ∼0.84 of TiO2 NRs actually agrees with the hypothesis of surface depletion regioncontrolled photoconduction behavior. Finally, according to the idea of the conventional oxygen sensitization model, the directly adsorbed oxygen molecule has been simply considered the origin of a surface acceptor or electron trap state. However, the potential influence by the residual water film on the TiO2 surface should also be included. Recently, Chakrapani et al. have pointed out that the oxygen electrochemical redox couple is more energetically favorable to the charge transfer between the atmosphere and the solid, especially for the wide-bandgap semiconductors.44,45 Although the direct exposure to water vapor of ambient air might be avoidable by the measurement in pure dry oxygen, it is still difficult to rule out the contribution of the residual water film on the material surface by the measurements at room temperature in this study. To clarify this point, more studies are required and will be elaborated in the future.

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CONCLUSIONS Photoconduction properties of the single-crystalline TiO2 NRs have been studied systematically. Both the high saturation Γ and especially the Γn have been obtained for the TiO2 NRs. By ruling out the contributions of experimental parameters, the maximal Γn at the saturation level is slightly higher than that reported for the ZnO NWs and is over 4 orders of magnitude higher than that of the polycrystalline TiO2 NTs. The longlifetime spatially separated carrier transport induced by the surface band bending in this titania nanostructure is proposed to contribute the highly efficient photoconduction in addition to the conventional oxygen sensitization mechanism. Finally, the study indicates the single-crystalline TiO2 NRs can be a highly efficient transport media for carriers, which could benefit the applications of DSSC, photodetectors, and photochemical devices.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author R.S.C. would like to thank the financial support of the Taiwan National Science Council (Grant No. NSC 994271

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