Chinese Journal of Chemical Physics  2016, Vol. 29 Issue (6): 687-692

#### The article information

Yu-kun Wu, A-wei Zhuang, Chun-miao Ye, Jie Zeng, Nan Pan, Xiao-ping Wang

Effect of Screw-Dislocation on Electrical Properties of Spiral-Type Bi2Se3 Nanoplates

Chinese Journal of Chemical Physics, 2016, 29(6): 687-692

http://dx.doi.org/10.1063/1674-0068/29/cjcp1605107

### Article history

Accepted on: July 25, 2016
Effect of Screw-Dislocation on Electrical Properties of Spiral-Type Bi2Se3 Nanoplates
Yu-kun Wu, A-wei Zhuang, Chun-miao Ye, Jie Zeng, Nan Pan, Xiao-ping Wang
Dated: Received on May 12, 2016; Accepted on July 25, 2016
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Physics, University of Science and Technology of China, Hefei 230026, China
*Author to whom correspondence should be addressed. Xiao-ping Wang, E-mail:xpwang@ustc.edu.cn
Abstract: We systematically investigated the electrical properties of spiral-type and smooth Bi2Se3 nanoplates through field effect transistor and conductive atomic force microscopy (CAFM) measurement. It is observed that both nanoplates possess high conductivity and show metallic-like behavior. Compared to the smooth nanoplate, the spiral-type one exhibits the higher carrier concentration and lower mobility. CAFM characterization reveals that the conductance at the screw-dislocation edge is even higher than that on the terrace, implying that the dislocation can supply excess carriers to compensate the low mobility and achieve high conductivity. The unique structure and electrical properties make the spiral-type Bi2Se3 nanoplates a good candidate for catalysts and gas sensors.
Key words: Bi2Se3 nanoplates    Screw-dislocation    Electrical properties    Field effect transistor    Conductive atomic force microscopy
Ⅰ. INTRODUCTION

Two dimensional (2D) layered materials, such as graphene, transition-metal dichalcogenides (TMDs) and Bi2Se3, have attracted extensive interest in the past decade [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Bi2Se3, a material with a planar quintuple layer (QL) in one unit cell, is well known for the thermoelectric properties due to its high figure of merit (ZT) [12, 13, 14]. Also, it is confirmed recently to possess topological insulator (TI) gapless surface states protected by time-reversal symmetry. This makes Bi2Se3 a outstanding candidate for the next generation of three dimensional (3D) TI [13, 15, 16]. When a bulk material becomes into a nanomaterial, its physical properties can be affected drastically by its size, structure, composition and defect. Specifically, by introducing screw dislocation as the driving force, the symmetry of crystal growth can be broken, resulting in the formation of anisotropic nanostructures [17, 18, 19]. By now, a number of nanomaterials have been synthesized through screw-dislocation-driven (SDD) growth mechanism, including metallic and semiconductor nanowires [18, 20, 21, 22, 23, 24, 25], nanotubes [21], nanoplates [26, 27, 28], nanoflowers [29], and nanotrees [18, 22]. However, compared to the research on the preparation of unique nanostructures as well as their growth mechanisms, few investigation has been reported on the effect of screw-dislocation on their physical properties, especially the electrical and optical characteristics.

Herein, by utilizing field effect transistor (FET) and conductive atomic force microscopy (CAFM) characterization on both spiral-type and smooth Bi2Se3 nanoplates (NPs), we systematically investigated the effect of screw-dislocation on the electrical transport properties of the layered Bi2Se3. The spiral-type Bi2Se3 and smooth NPs are prepared with polyol method through SDD growth mode and layer-by-layer growth mode, respectively [30]. It is observed that both kinds of Bi2Se3 FETs show high conductivity and weak gate-modulation behavior. The conductivity of spiral-type Bi2Se3 NP is higher than that of smooth one. Also, the resistances of both NPs increase with increasing temperature, indicating the metallic-like feature. More importantly, the spiral-type NP is found to possess higher carrier concentration and lower electron mobility as compared to the smooth one. With CAFM characterization on the spiral-type sample, the spatial distribution of conductivity can be observed and the contribution of screw-dislocation to its conductance is identified. We suggest that the dislocation in the spiral-type Bi2Se3 NPs can supply excess carriers, which can compensate the low mobility and enhance its conductivity.

Ⅱ. EXPERIMENTS

Spiral-type and smooth Bi2Se3 nanoplates (NPs) were synthesized by a polyol method. Details can be found in our previous report [30]. Briefly, solutions of Bi (NO3)35H2O and Na2Se3 in ethylene glycol (EG) were simultaneously injected into another EG mixed with poly (vinylpyrrolidone) (PVP), L-ascorbic acid (AA) and hydro-chloric acid (HCl) at 175 ℃ under N2 atmosphere. At lower precursor concentration, screw-dislocation-driven (SDD) growth mode was dominated and spiral-type NPs were prepared. When tuning the super-saturation to a higher level, e.g., 0.5 times larger, layer-by-layer growth mode occurred and smooth NPs could be formed.

To fabricate FETs for both NPs, the as-grown product dispersed in an EG solution was drop casted onto Si substrates covered with a 300-nm-thick SiO2 layer. Standard electron-beam lithography (EBL) process by a Raith e-Line system was employed, and 10-nm-thick Ti and 100-nm-thick Au were deposited as the electrodes.

The morphologies of both NPs were characterized by field-emission scanning electron microscopy (FE-SEM, JEOL-6700 or Sirion200) and atomic force microscopy (AFM, SPA 300HV). The electrical measurements of FETs were carried out using a semiconductor characterization system (Keithley 4200) in a probe station (LakeShore CRX-4K) or in a homemade chamber. The spatial conductance features of the NPs were characterized by CAFM (SPA 300HV) in contact mode with a conductive Pt/Cr/Si tip. The NPs were transferred onto a conducting substrate, either highly oriented pyrolytic graphite (HOPG) or Au/Si substrate, and voltage bias was applied to the substrate with respect to the tip. All the electrical and CAFM results were performed in vacuum.

Ⅲ. Results and discussion

Figure 1(a) and (b) show the representative SEM images of as-grown spiral-type and smooth Bi2Se3 NPs, respectively. As seen, both NPs have hexagonal morphologies, with the diameter ranging from 0.8 µm to 3.2 µm for the spiral-type NPs and from 4 µm to 8 µm for the smooth samples. The insets of Fig. 1(a) and (b) show that the spiral-type NP contains helical fringes on the surface, while the smooth one has flat surface. The morphology and the thickness of both NPs were further characterized by AFM. Figure 1(c) and (d) illustrate the AFM topography images of the typical spiral-type NP and the smooth one, and Fig. 1(e) and (f) give the height line profiles marked in Fig. 1(c) and (d), respectively. As seen, the spiral-type NP exhibits a dislocation hillock, with the center height of 63 nm and edge height of~35 nm, confirming the spiral-type structure. In contrast, the smooth NP has a uniform height of 5 nm, in accord with the SEM results.

 FIG. 1 Morphologies of Bi2Se3 nanoplates. (a, b) SEM images of (a) spiral-type and (b) smooth Bi2Se3 nanoplates. Insets show the high-magnification SEM image of a representative nanoplate. (c, d) AFM image of an individual (c) spiral-type and (d) smooth Bi2Se3 nanoplate, respectively. (e, f) Corresponding height line profiles of the (e) spiral-type and (f) smooth nanoplate on the locations marked with line in (c) and (d).

To investigate the effect of screw-dislocation on the electrical properties of layered Bi2Se3, FETs for both NPs are fabricated and typical SEM images of the devices are shown in Fig. 2(a) and (b). The channel length are kept the same as 0.8 µm for both NP FETs. The electrical measurements for the devices are performed in a probe station at various temperature. Figure 2(c) show $I-V$ curves of both FETs at 5 K and at room temperature, respectively. Two features can be observed. First, all $I-V$ curves demonstrate the linear feature, indicating good ohmic contact between the sample and the electrodes. Second, the conductance of the spiral-type NP is larger than that of the smooth one, and it varies weakly with temperature from 5 K to room temperature. Figure 2(d) shows the corresponding transfer characteristics of both FETs, revealing the source-drain current ( $I_{\rm{sd}}$ ) responses to the gate voltage ( $V_{\rm{g}}$ ). As seen, for the smooth NP FET, $I_{\rm{sd}}$ shows a small decrease by sweeping $V_{\rm{g}}$ from +40 V to -30 V, demonstrating the n-type conduction; however, no modulation can be observed for the spiral-type one, implying that the spiral-type sample has much higher carrier concentration.

 FIG. 2 Electrical properties of both Bi2Se33 nanoplates. (a, b) SEM images of FET devices for (a) spiral-type and (b) smooth Bi2Se33 nanoplate. (c) Typical I-V curves and (d) transfer characteristics for both Bi2Se33 nanoplate devices at 5 and 300 K, respectively.

It is well known that the field effect mobility ( $\mu$ ) and carrier concentration (n) of a FET can be obtained using the following equations:

 $\mu =\frac{{{L}^{2}}}{C{{V}_{\text{sd}}}}\cdot g=\frac{{{L}^{2}}}{C{{V}_{\text{sd}}}}\cdot \frac{\text{d}{{I}_{\text{sd}}}}{\text{d}{{V}_{\text{g}}}}$ (1)
 $C=\frac{{{\varepsilon }_{0}}{{\varepsilon }_{\text{d}}}S}{h}$ (2)
 $n=\frac{\sigma }{q\mu }$ (3)

Here L is the channel length, C is the capacitance between the NP and the gate, $V_{\rm{sd}}$ is the applied source-drain voltage, g is the trans conductance obtained from the linear region in the transfer curve, $\sigma$ is the conductivity of the NP calculated from the $I-V$ curve, q=1.6×10-19 C is the elementary charge, $\varepsilon_0$ =8.85×10-12 F/m is the vacuum permittivity, $\varepsilon_{\rm{d}}$ is the relative static permittivity of the gate dielectric (3.9 for SiO2 here), h is the thickness of the gate dielectric (300 nm SiO2 here), and S is the channel area of the device. For the smooth NP FET, the electron concentration and mobility can be deduced as $n_\textrm{e} \approx$ 1.5×1020 cm-3 and $\mu_\textrm{e} \approx$ 17 cm-2/Vs. Unfortunately, the values of $n_\textrm{e}$ and $\mu_\textrm{e}$ for the spiral-type FET is unable to estimate from the above procedure because no gate modulation exists in the device. However, by comparing the electrical properties of spiral-type and smooth devices shown in Fig. 2(c) and (d), we reasonably speculate that the spiral-type NP has more electron concentration and lower mobility.

To verify this point, we further investigate the photoelectric response for both NP FETs. It is found that, at the same condition, the smooth NP FET has observable response to UV light, while there is no response for the spiral-type one (Fig. 3(a) and (b)). This demonstrates that the spiral-type NP has much higher intrinsic electron concentration than the smooth one. It is worthy to point out that the deduced carrier density 1.5×1020 cm-3 for the smooth NP is such high that the metal-insulator transition may occur in these Bi2Se3 NPs. In this regard, we carried out the measurement for the temperature-dependence of the resistance for both NPs, and the results are shown in Fig. 3(c) and (d). As seen, the resistances of both NPs increase monotonically with temperature, in accord with the temperature dependence of the metal, i.e., the Bi2Se3 NPs show the metallic-like electrical behavior.

 FIG. 3 Photoresponse of (a) spiral-type and (b) smooth Bi2Se3 nanoplate FETs. (c, d) The temperature dependence of the resistance for (c) spiral-type and (d) smooth Bi2Se3 nanoplate. The solid lines in (c) and (d) are the fitting curves using Eq.(4).

Following the previous studies [31, 32], we also fitted the results in Fig. 3(c) and (d) by the equation:

 $R = R_0+R_\textrm{i}=R_0+bT^\alpha$ (4)

where R0 and $R_\textrm{i}$ describe the residual resistance and ideal resistance of the NPs, and b and α are constants. It can be found that the fitting curves (solid lines in Fig. 3(c) and (d)) are well consistent with the experiment results. The fitting parameter values are summarized in Table Ⅰ. Note that, since R0 and $R_\textrm{i}$ are originated from the defect scattering and phonons scattering, respectively. The intrinsic quality of the NPs can be further evaluated by the residual-resistance ratio [33], $R_{\rm{RR}}$ = $R_{300}/R_0$ , where $R_{300}$ is the resistance at room temperature. Generally, the smaller the $R_{\rm{RR}}$ is, the more defects the sample has. As seen in Table Ⅰ, the spiral-type sample has smaller RRR compared to the smooth one, implying more contribution from the defect scattering in the spiral-type sample. This result is in agreement with the low mobility of the carriers for spiral-type NPs deduced from the transfer character shown in Fig. 2(d). Moreover, the parameter α for the spiral NP (~3.77) is much larger than that for the smooth one (~1.77), also implying more defect existing in spiral NPs [32].

Table Ⅰ Fitting parameters for curves in Fig.3 (c) and (d) using Eq.(4).

Based on the above electrical characterizations on the Bi2Se3 NPs with different morphologies, we have found that, as compared to the smooth NP, the spiral-type one exhibits the higher carrier concentration and lower electron mobility. While the low mobility can be rationally understood by the more defect scattering from the screw-dislocation in the spiral-type NP, the relationship between the higher carrier concentrations and the screw-dislocation is still in doubt. To this end, we utilize CAFM characterization to probe the spatial distribution of the conductance for both NPs. Figure 4(a) and (b) show typical topography images for a spiral-type and smooth NP, and Fig. 4(c) and (d) are their corresponding current maps, respectively. As seen in the CAFM images, the smooth NP has nearly the same current on its surface, revealing the uniformly electrical properties. In contrast, the screw-like current patterns can be observed on the surface of the spiral-type sample. More importantly, the patterns are located in good line with the dislocation terraces. This can be further identified from the line profiles obtained from both the topographic and current maps, as shown in Fig. 4(e). It is clearly seen that the high-current peaks appear exactly right at the terrace edges (marked as red arrows) and at the pits (labeled as blue crosses).

 FIG. 4 CAFM characterizations of Bi2Se3 nanoplates. (a, b, d, e) AFM images of an individual (a) spiral-type and (d) smooth Bi2Se3 nanoplate, with the corresponding current images shown in (b) and (e), respectively. (c) Topography as well as current line profiles for the spiral-type sample obtained along the black lines in (a) and (b), respectively. (f) I-V curves acquired at different locations marked as crosses in (a) and (d).

In order to obtain more detailed information of the local electrical properties, the Ⅰ-Ⅴ curves were recorded with the CAFM tip located at four different positions: the conducting substrate, the surface of the smooth NP, the terrace, and the terrace edge of the spiral-type NP. The results are presented in Fig. 4(f). As seen, the current at the substrate (sp4) saturates quickly with the bias, indicating its good conductivity. For the NPs, both the currents at the terrace and at the terrace edge of the spiral-type NP are much larger than that at the surface of the smooth NP, in accord with the above FET characterization. Furthermore, the conductance at different points are estimated: the smooth NP (sp3)~4 nS; the terrace (sp1) of the spiral-type NP~16 nS, and the terrace edge (sp2)~80 nS. Therefore, we can conclude that the terrace edge has enhanced conductance and its conductance is almost 20 times larger than that for the smooth NP.

In the CAFM measurement, two artificial factors which should be considered before the intrinsic cause of the current enhancement at the terrace edge can be identified: the changes in contact geometry and area, and the local removal of oxides or surface contaminates. As known in AFM, the tip geometry and sample surface morphology are convoluted, which can produce artifacts in the current map due to changes in contact area. In fact, from Fig. 4(e) we can see that the current is enhanced both at the terrace edges and at the pits. It is apparent that the contact area is larger when the tip probes the pits. However, a relative small (30%-60%) increase in current is seen at the pits, and up to 3 times enhancement at the terrace edges. Similarly, Macedo et al. [34] characterized the local electrical properties of MBE-grown Bi2Te3 film by CAFM, and found a magnification of 10-100 enhancement in current at step-edges relative to that on the terraces. For comparison, they performed CAFM on two control samples, i.e., two dimensional HOPG and Fe3O4 nanoparticles (with the diameter of~20 nm)/graphene/SiO2/Si. No more than 20% enhancement were observed for both sample at either the step-edges of HOPG or the edges of the Fe3O4 nanoparticles. Thus, we can exclude the changes of contact area as the main reason for the current enhancement at the terrace edges.

Another factor is the local removal of oxides or surface contaminates at the terrace edges by tip scanning. In our CAFM measurements, the spiral-type NP was scanned from left to right, leading to the tip-NP interaction being weaker at falling steps (right-half) than at rising steps (left-half) and on terraces. However, we observe enhanced currents at both rising and falling steps of the terraces (Fig. 4(c)). Besides, the line profiles were obtained at the falling steps as well (Fig. 4(e)). So this factor can also be ruled out. Therefore, we believe that the current enhancement at the terrace edges is contributed from the difference between the local electrical properties at the terrace edge and on the terraces, i.e., the high carrier concentrations in the spiral-type NP is dominantly contributed by the screw-dislocation.

To further testify the contribution of screw-dislocation, we also investigate the relationship between the conductivity and the dislocation densities for several spiral-type NPs. The density of screw-dislocation for a sample is calculated by the following equation [35],

 $\rho= \frac{L}{V}=\frac{\displaystyle\sum l}{V}$ (5)

where L is the sum of the length l for each dislocation, and V is the sample volume. We counted all the dislocation lengths for four spiral-type NP FETs from the SEM images, and calculated their dislocation densities. The corresponding conductivities of the devices were obtained from the electrical measurement, and the conductivity dependence on the dislocation density is plotted in Fig. 5. As seen, the conductivity increases first with the dislocation density up to~1200 µm-2. This is the strong evidence that the screw-dislocation does contribute to the high conductivity of the spiral-type NPs via supplying more carriers. Also from Fig. 5, we find that, when the dislocation density is beyond~1200 µm-2, the conductivity of the sample become decreasing, which can be possibly ascribed to the severe defect scattering from the dislocation. The underlying mechanism of the behavior needs further research.

 FIG. 5 Relationship of the conductivity to the dislocation density for the spiral-type Bi2Se3 nanoplates at 5 K and at room temperature.
Ⅳ. CONCLUSION

In summary, we systematically and comparatively investigated the electrical properties of both spiral-type and smooth Bi2Se3 NPs. Electrical measurements show that both NPs have high conductivities and exhibit metallic-like behavior. However, compared with the conventional smooth NPs, the spiral-type NPs display higher carrier concentration and lower electron mobility. Using CAFM, the characterization of the spatially conductive distribution reveals a higher conductance at the terrace edge in the spiral-type NPs. Furthermore, we find that, although the screw-dislocation in the spiral-type NPs causes defect scattering and lower the carrier mobility, it can supply more carriers and lead to the high conductivity. This unique structure and high conductivity of spiral-type Bi2Se3 NPs might find special application for catalysis and gas sensors in the future.

Ⅴ. ACKNOWLEDGMENTS

This work is supported by the Ministry of Science and Technology of China and the National Natural Science Foundation of China.

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