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.

Intrinsic carrier transport properties of single-walled carbon nanotubes are probed by two parallel methods on the same individual tubes: the contactless dielectric force microscopy (DFM) technique and the conventional field-effect transistor (FET) method. The dielectric responses of SWNTs are strongly correlated with electronic transport of the corresponding FETs. The DC bias voltage in DFM plays a role analogous to the gate voltage in FET. A microscopic model based on the general continuity equation and numerical simulation is built to reveal the link between intrinsic properties such as carrier concentration and mobility and the macroscopic observable, i.e. dielectric responses, in DFM experiments. Local transport barriers in nanotubes, which influence the device transport behaviors, are also detected with nanometer scale resolution.

Nanomaterials are increasingly used in electronic, optoelectronic, bioelectronic, sensing, and energy nanodevices. Characterization of electrical properties at nanometer scales thus becomes not only a pursuit in basic science but also of widespread practical need. The conventional field-effect transistor (FET) approach involves making electrical contacts to individual nanomaterials. This approach faces serious challenges in routine characterization due to the small size and the intrinsic heterogeneity of nanomaterials, as well as the difficulties in forming Ohmic contact with nanomaterials. Since the charge carrier polarization in semiconducting and metallic materials dominates their dielectric response to external fields, detecting dielectric polarization is an alternative approach in probing the carrier properties and electrical conductivity in nanomaterials. This Account reviews the challenges in the electrical conductivity characterization of nanomaterials and demonstrates that dielectric force microscopy (DFM) is a powerful tool to address the challenges. DFM measures the dielectric polarization via its force interaction with charges on the DFM tip and thus eliminates the need to make electrical contacts with nanomaterials. Furthermore, DFM imaging provides nanometer-scaled spatial resolution. Single-walled carbon nanotubes (SWNTs) and ZnO nanowires are used as model systems. The transverse dielectric permittivity of SWNTs is quantitatively measured to be ∼10, and the differences in longitudinal dielectric polarization are exploited to distinguish metallic SWNTs from semiconducting SWNTs. By application of a gate voltage at the DFM tip, the local carrier concentration underneath the tip can be accumulated or depleted, depending on charge carrier type and the density of states near the Fermi level. This effect is exploited to identify the conductivity type and carrier type in nanomaterials. By making comparison between DFM and FET measurements on the exact same SWNTs, it is found that the DFM gate modulation ratio, which is the ratio of DFM signal strengths at different gate voltage, is linearly proportional to the logarithm of FET device on/off ratio. A Drude-level model is established to explain the semilogarithmic correlation between DFM gate modulation ration and FET device on/off ratio and simulate the dependence of DFM force on charge carrier concentration and mobility. Future developments towards DFM imaging of charge carrier concentration or mobility in nanomaterials and nanodevices can thus be expected.