Contactless Characterization of Electronic Properties of Nanomaterials

Mar 5, 2012 - Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei Anhui 230026, Ch...
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Contactless Characterization of Electronic Properties of Nanomaterials Using Dielectric Force Microscopy Wei Lu,† Jie Zhang,†,‡ Yize Stephanie Li,† Qi Chen,†,‡ Xiaoping Wang,‡ Abdou Hassanien,§,∥ and Liwei Chen*,† †

i-LAB, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei Anhui 230026, China § Electronics and Photonics Research Institute, AIST, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan ∥ National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia ‡

S Supporting Information *

ABSTRACT: Characterization of electronic properties of nanomaterials usually involves fabricating field effect transistors and deriving materials properties from device performance measurements. The difficulty in fabricating electrical contacts to extremely small-sized nanomaterials as well as the intrinsic heterogeneity of nanomaterials makes it a challenging task to measure the electronic properties of large numbers of individual nanomaterials. Here, we utilize a scanning probe technique, the dielectric force microscopy (DFM) to address the challenges. The DFM technique measures the low frequency dielectric response of nanomaterials, which is intrinsically related to their electrical conductivity. The incorporation of a gate bias voltage in DFM measurements allows for charge carrier density modulation, which is exploited to determine the carrier type in nanomaterials such as semiconducting single-walled carbon nanotubes (SWNTs) and ZnO nanowires (ZnO NWs). This technique avoids the need of electrical contacts and inherits the spatial mapping capability of scanning probe microscopy, as manifested in the imaging of intratube metallic/semiconducting junctions in SWNTs. We expect the DFM technique to find broad applications in the characterization of various nanoelectonic materials and nanodevices.

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INTRODUCTION Because of their high specific surface area and/or quantum confinement effects, nanomaterials such as nanoparticles, nanotubes, nanowires, and nanosheets exhibit electronic properties that are highly dependent on their size and structure. This on one hand makes nanomaterials intriguing building blocks for bottom-up assembly of nanoelectronic devices because of the tunable properties, which are different from their bulk form; but on the other hand, variation in size, composition, impurity, doping, and other structural parameters results in heterogeneity in their electronic properties. Characterizing the electronic properties of nanomaterials has been a standing challenge, with the most common approach being fabricating field-effect transistors (FETs) and deriving the material properties from transistor performance data.1 While this approach has been successful in fundamental research, it requires a nanofabrication facility and is also limited by the low throughput. Furthermore, transport characteristics of many devices are dominated by metal contacts and heavily influenced by testing environments,2−4 making it difficult to unveil the intrinsic properties of nanomaterials. Toward addressing this challenge, here we report a scanning probe technique that characterizes the electronic properties of nanomaterials without the need of metal contacts. © 2012 American Chemical Society

Electrostatic force microscopy (EFM) has been widely used to investigate electrostatic properties of nanomaterials. In EFM experiments, an AC bias voltage with frequency ω is applied between a conducting atomic force microscope (AFM) probe and the sample. A lock-in amplifier at the same AC frequency ω is used to detect interaction forces between the tip and the electrostatic field due to static charges and/or permanent dipole moments of the sample. In addition, a dipole moment in the sample induced by the AC bias voltage also interacts with the tip. This dielectric force oscillates at the double frequency and is thus detected using a separate lock-in amplifier at the 2ω frequency. The electrostatic force (i.e., 1ω) channel in EFM has been utilized in investigating nanometer-scaled distribution of charge and electric dipole moment, surface potential variation, charge storage, and leakage in materials and devices.5−9 The 2ω channel in EFM has also been utilized to measure the equivalent dielectric constant of individual SWNTs.10,11 Here, we specifically dub the 2ω channel in EFM as dielectric force microscopy (DFM) to emphasize its application in electronic property measurements. The dielectric response of nanomaterials deserves particular investigation due to its Received: January 21, 2012 Revised: February 29, 2012 Published: March 5, 2012 7158

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correlation with the electronic conductivity. For ideal metals, their complex low frequency dielectric constant is a simple function of the electronic conductivity σ, as described in the Drude model, ε ≈ ε0(1+i(σ/ω)) ≈ iε0(σ/ω), where ω is the angular frequency of the external AC field.12 For semiconductors, the dielectric response due to the motion of charge carriers dominates over the polarization of the lattice, which suggests that dielectric response and electric conductivity are both determined by the carrier density and mobility. Consequently, the conductivity of nanoelectronic materials could be indirectly probed through DFM measurement. In this article, we demonstrate the potential of DFM as a contactless method for the characterization of nanoelectronic materials using single-walled carbon nanotubes (SWNTs) and ZnO nanowires (ZnO NWs) as model systems. In order to identify the carrier type in these nanomaterials, we add a DC component to the DFM tip voltage, which modulates the carrier density of the local section of nanotubes or nanowires right below the DFM probe. The dielectric response is then measured as an indication of electronic conductivity to identify, whether the local section of the nanomaterial displays metallic or semiconducting properties, and in the semiconducting case, whether it is in the carrier accumulation or depletion regime. A unique feature of DFM is that the measurement requires no metal contacts to the nanomaterial and offers spatial mapping capability with nanometer-scaled resolution. This is demonstrated by the observation of intratube metal−semiconductor junctions in individual SWNTs. We believe that the DFM technique will find wide application in nanomaterials and device research.

Figure 1. Schematic illustration of DFM experiments. The nanomaterial samples are imaged in a modified double-pass imaging process. In the first pass, the standard AC mode AFM imaging is performed on the sample to obtain a topographic scan line; in the second pass, the oscillation of the cantilever at resonance frequency is turned off, and the tip is lifted up to a certain height and scans in a trace parallel to the topographic baseline obtained in the first scan. A bias voltage V = Vzero + Vg + Vac sin(ωt) is applied between the conductive tip and the Si substrate only in the second pass.

component to zero out the contact potential difference between the substrate and the probe9 (typically within ±0.1 V in this experiment); and Vg was the DC voltage component that modulated the local carrier density in the sample, which was varied between −4 V and +4 V in this experiment. The AC frequency ω (typically 10k Hz) was set to be far below the cantilever resonance, thus only forced oscillation driven by the electrostatic interactions were measured in the second pass. The AC bias in ω frequency between the tip and the substrate polarized the sample and resulted in an induced dipole moment also oscillating in ω frequency. Since both the charges on the conducting AFM tip and the induced dipole on the sample were proportional to sin(ωt), the interaction force between the two becomes proportional to sin2(ωt) = 1/2 − 1/2 cos(2ωt), which is an attractive force oscillating in 2ω frequency.10,11 Two Stanford SR830 lock-in amplifiers (Stanford Research Systems, Inc., Sunnyvale, CA) were used to collect the ω and 2ω components of the cantilever deflection signal with a 3 ms integration time. The ω component of the deflection was the conventional EFM signal, which was only used to determine the Vzero in this study (Vzero is the DC bias voltage at which the 1ω channel EFM force became zero). The 2ω component was the dielectric response signal extensively studied in this work and was fed back to the data acquisition software of the AFM system to generate DFM images. The direct force detection approach used in this experiment differs from the force gradient detection approach in conventional EFM. In the force detection method, the obtained DFM signal can be converted to the dielectric interaction force between the sample and the probe once the cantilever spring constant is calibrated, which provides a more straightforward approach for potential quantitative modeling compared with the force gradient-based detection method used previously.6

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EXPERIMENTAL SECTION Sample Preparation. SWNTs prepared by the laser ablation method were dispersed in 1,2-dichloroethane by a gental sonication for 10−15 min. The fresh-prepared dispersion was spin-coated on a p+2-Si wafer with 50 nm thermal oxide. Vertical aligned ZnO nanowires grown by CVD were transferred onto Si wafer with a piece of flat and pattern-free PDMS stamp. SWNT and ZnO NW on Si wafer samples were assembled into an enclosed fluid cell (Park Universal Liquid Cell, Park Systems Corp.). A continuous nitrogen gas flow ( 0, where holes are depleted in the region below the tip. Here, Ec and Ev are the conduction and valence band edge, respectively; EF and Ei denote the Fermi level and the midgap energy level, respectively.

redistribution of carriers within the nanotube due to the tip voltage (Figure 3). The DFM technique as a contactless local probe of the conductivity for generic nanoelectronic materials is further demonstrated on ZnO nanowires, as shown in Figure 1i−l. The strengthening of the dielectric response as the gate voltage increases from −2 to 2 V (also shown quantitatively in Figure 1m), which is just opposite to the behavior of semiconducting SWNTs, indicates the n-type nature of the ZnO nanowire, which is consistent with FET transport measurements reported before.22,23 Spatial Mapping Capability of DFM. An important advantage of DFM as a contactless local conductivity measurement technique is the nanometer-scaled spatial resolution inherent to scanning probe microscopy techniques. Again, we use SWNTs as the model system to demonstrate this important feature. Figure 4 shows intratube metal−semiconductor (M−S) junctions in individual SWNTs. The topographical image in Figure 4a shows an individual SWNT with regular morphology, but the dielectric response clearly identifies it as an M−S intratube junction (Figure 4b−d), with the upper part of the tube being metallic and the lower part 7160

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Figure 4. Topographical (a) and dielectric response (b−d) images of a SWNT with an M−S intratube junction at Vg = −2 V, 0 V, and 2 V. Topographical (e) and dielectric response (f−h) images of a SWNT with an S−M−S intratube junction at Vg = −2 V, 0 V, and 2 V. The center position of image (h) was manually adjusted to offset scanner drift.

being semiconducting. Figure 4f−h shows that the SWNT in Figure 4e has two M−S intratube junctions, with the middle section of the tube (∼1 μm in length) being metallic and the top and bottom sections being semiconducting. Similar intratube M−S junctions are frequently observed in our DFM experiments regardless of the origin of the SWNT samples from HiPco, CoMoCAT, or laser ablation. We thus believe that the DFM technique can be an important characterization tool for controlling the properties of SWNTs in the growth processes.24−26 One can imagine that such junctions would be difficult to identify, not to mention to be spatially located using electrical transport measurements. A micro-Raman imaging technique had been used to detect junctions in SWNTs,24 but the spatial resolution is about 1 μm, much lower than that of the DFM technique (tens of nanometers). Furthermore, micro-Raman imaging is capable of detecting only those SWNTs in resonance with the excitation laser, which greatly limits its application in SWNT studies. Metallic/Semiconducting SWNT Contrast under Different Gate Bias. Previously, we have reported a EFM-based assay for the metallicity of SWNT samples in which the semiconducting and metallic tubes cluster into two zones separated by a gap in the dielectric response versus diameter square (D2) plot.11 We notice that in Figure 2m, the dielectric response signal contrast between SWNT A and SWNT B is much greater under a positive Vg than zero Vg. Thus, intuitively, a direction of improvement over the previous assay is to measure the dielectric response under a positive bias, which shall result in a wider gap between the metallic and semiconducting zones. Figure 5a−c show the dielectric force vs D2 plots of a set of 18 SWNTs measured under different Vg. When Vg = −2 V, semiconducting SWNTs are in the accumulation regime with a high density of holes under the DFM tip and thus show dielectric forces comparable to those of metallic SWNTs. The two types of nanotubes as represented by SWNT A and SWNT B cannot be distinguished in this case (Figure 5a). When Vg = 0 V, hole density in semiconducting SWNTs is reduced, and consequently, semiconducting tubes display smaller dielectric forces than metallic ones. However, the difference is not significant, and the 18 data points are not sufficient to define a clear gap (Figure 5b). At Vg = 2 V, holes in semiconducting tubes are further depleted, and as a result, the dielectric forces are significantly reduced. Figure 5c shows a clear gap of more than 3 pN in width between the metallic and semiconducting zones. By measuring the dielectric responses of

Figure 5. (a−c) Dielectric force versus diameter square plots of 18 SWNTs at Vg = −2 V, 0 V, and 2 V, respectively. Here, A and B denote data points from the metallic and semiconducting SWNTs A and B shown in Figure 2, respectively. SWNT C is a tube that falls into the semiconducting zone, but its dielectric response signal is insensitive to different gate bias.

SWNTs under a proper positive bias voltage, dielectric polarization contribution from free carriers in semiconducting SWNTs is suppressed, while that in metallic SWNTs remains roughly unchanged, thus achieving a high contrast in the dielectric response vs D2 plot. New SWNT Metallicity Assay Defined by the DFM Gate Modulation Ratio. While the SWNT metallicity assay above has been verified with numerical modeling and singletube resonance Raman imaging and spectroscopy techniques,11 it still has some drawbacks. A major problem is that the assignment of the metallic and semiconducting zones relies on the collective clustering of data points, which requires a large number of both types of SWNTs measured with the same AFM tip. An ideal assay for the metallicity of individual SWNTs shall interrogate the property of an individual tube itself only, without relying on other tubes or being affected by tip variation and abrasion. We propose to use a gate modulation ratio, i.e., the ratio of the DFM signal at different gate bias voltages, for example, at Vg = 2 V to that at Vg = −2 V (S2V/S−2V), as an indication of the electronic property of SWNTs. This ratio is a measure of the gatebility of charge carriers, i.e., the degree to which the charge carrier density is modulated by the gate bias. Figure 6a shows the dielectric signal gate modulation ratio of the 18 SWNTs previously analyzed in Figure 5. As expected, there is no correlation between this ratio and the tube diameter (Figure 6a). The SWNTs identified as metallic in Figure 5c exhibit a gate modulation ratio in the 0.9−1.2 range, while the semiconducting ones are in the range of 0.3−0.8. Gate modulation ratio (S2V/S−2V) values close to 1 indicate bias independent carrier density and thus imply the metallic behavior; and gate modulation ratio values less than 1 indicate accumulation/depletion of carriers right below the tip under a negative/positive local gate voltage, which is characteristic of ptype semiconducting materials. Following this principle, we 7161

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signal does not change much at different gate bias, giving rise to a gate modulation ratio of 1.1, which is characteristic of metallic tubes. The relatively low conductivity of SWNT C might be a result of either low carrier density or low carrier mobility. Besides manifesting real material properties, the gate modulation ratio method also has additional practical advantages over the previously published metallicity assay.11 First, the electronic type of an individual SWNT is determined by measuring the dielectric response of the particular SWNT itself rather than measuring lots of SWNTs to get the metallic and semiconducting zones. Second, obtaining the gate modulation ratio needs the original DFM signal strength only, so it is unnecessary to convert to the quantitative dielectric force between the tip and the SWNTs. Third, the data measured by different tips with different tip apex geometries can be used in one set of measurement without calibration and normalization, so the wearing of the tip is no longer a limiting factor for the DFM-based metallicity assay, and a large number of SWNTs samples can be measured to get reliable statistic results. In summary, we demonstrate that the DFM is a powerful tool for contactless characterization of electronic properties of nanomaterials. The incorporation of an adjustable gate bias in DFM imaging enables active modulation of charge carrier densities in the sample while indirectly monitoring the conductivity through dielectric response imaging. This technique is uniquely well suited to address the challenges in nanoelectronic material characterization arisen from their small size and intrinsic heterogeneity. Our results on SWNTs and ZnO NWs forecast much broader applications in nanomaterials and devices research.

Figure 6. (a) Plot of gate modulation ratio of the dielectric responses of SWNTs at Vg = 2 V and −2 V, against tube diameter. A and B denote data points from the metallic and semiconducting SWNTs A and B shown in Figure 2, respectively. Data point C is from the same SWNT C as in Figure 5. (b) Histogram of the gate modulation ratio for the 18 SWNTs studied in Figure 5.

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expect that n-type semiconducting nanomaterials should show a gate modulation ratio (S2V/S−2V) that is greater than 1, as verified by the dielectric signal vs Vg trace for a ZnO NW in Figure 2m. Further experiments indicate that the gate modulation ratio is independent of lift height (Supporting Information, Table S1 and Figure S2). In addition, we found that DFM tips from different vendors with different resonance frequency and spring constant give a consistent gate modulation ratio (S2V/S−2V) for a given SWNT (Supporting Information, Figure S3, S4 and Table S2). Since the gate modulation ratio is an indication of the intrinsic electronic property of materials, the scattering of the experimental data from different SWNTs could be a reflection of the inherent complexity and heterogeneity of individual tubes. Figure 6b shows a histogram of the gate modulation ratio obtained from the 18 SWNTs studied in Figure 5. The two peaks centered around 0.5 and 1.0 represent the semiconducting and metallic regions, respectively. Interestingly, there are nanotubes with a gate modulation ratio falling in between, for example, at S2V/S−2V = 0.8. It is inappropriate to assign these tubes to be either metallic or semiconducting. The ambiguous gate modulation ratio value suggests that the gatebility of charge carriers in these tubes is somewhere between that of ideal metals and ideal semiconductors, which could originate from SWNTs with structural variations, such as semimetallic tubes, metallic tubes with higher defect densities, or semiconducting tubes with small band gaps. Another example is the SWNT C in Figures 5 and 6. The magnitude of the dielectric response signal of SWNT C falls into the semiconducting zone at Vg = 2 V (Figure 5c). However, the

ASSOCIATED CONTENT

* Supporting Information S

Supporting data in Tables and Figures, discussion on the lift height dependence of DFM signal, lift height and tip dependences of the DFM gate modulation ratio. 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 work was supported by the National Natural Science Foundation of China (Nos:10904105, 20973122, and 11034001), the Knowledge Innovation Program of the Chinese Academy of Sciences (No. KJCX2-YW-H21), and the National Basic Research Program of China (2010CB934700). L.C. thanks the Chinese Academy of Science “Hundred Talents” program.

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