Dielectric Force Microscopy Research Articles

Identification of the conductivity type of single-walled carbon nanotubes via dual-modulation dielectric force microscopy.

The conductivity type is one of the most fundamental transport properties of semiconductors, which is usually identified by fabricating the field-effect transistor, the Hall-effect device, etc. However, it is challenging to obtain an Ohmic contact if the sample is down to nanometer-scale because of the small size and intrinsic heterogeneity. Noncontact dielectric force microscopy (DFM) can identify the conductivity type of the sample by applying a DC gate voltage to the tip, which is effective in tuning the accumulation or depletion of charge carriers. Here, we further developed a dual-modulation DFM, which simplified the conductivity type identification from multiple scan times under different DC gate voltages to a single scan under an AC gate voltage. Taking single-walled carbon nanotubes as testing samples, the semiconducting-type sample exhibits a more significant charge carrier accumulation/depletion under each half-period of the AC gate voltage than the metallic-type sample due to the stronger rectification effect. The charge carrier accumulation or depletion of the p-type sample is opposite to that of the n-type sample at the same half-period of the AC gate voltage because of the reversed charge carrier type.

Interlayer Charge Transfer Induced Electrical Behavior Transition in 1D AgI@sSWCNT van der Waals Heterostructures.

The emergence of one-dimensional van der Waals heterostructures (1D vdWHs) opens up potential fields with unique properties, but precise synthesis remains a challenge. The utilization of mixed conductive types of carbon nanotubes as templates has imposed restrictions on the investigation of the electrical behavior and interlayer interaction of 1D vdWHs. In this study, we efficiently encapsulated silver iodide in high-purity semiconducting single-walled carbon nanotubes (sSWCNTs), forming 1D AgI@sSWCNT vdWHs. We characterized the semiconductor-metal transition and increased the carrier concentration of individual AgI@sSWCNTs via sensitive dielectric force microscopy and confirmed the results through electrical device tests. The electrical behavior transition was attributed to an interlayer charge transfer, as demonstrated by Kelvin probe force microscopy. Furthermore, we showed that this method of synthesizing 1D heterostructures can be extended to other metal halides. This work opens the door for the further exploration of the electrical properties of 1D vdWHs.

Improved electronic uniformity and nanoscale homogeneity in template-grown CsPbBr3 nanorods.

One-dimensional metal halide perovskites are among the most promising candidate materials for optoelectronic devices. However, the heterogeneity and fast degradation of perovskite nanowires (NWs) and nanorods (NRs) synthesized using conventional approaches impose a bottleneck for their optoelectronic applications. Recently, all-inorganic perovskite CsPbBr3 NRs with tailored dimensions, crafted using an amphiphilic bottlebrush-like block copolymer (BBCP) as nanoreactors, have demonstrated enhanced stabilities. Herein, we report the electronic investigation into these template-grown CsPbBr3 NRs using dielectric force microscopy (DFM), a contactless, nondestructive imaging technique. All freshly prepared CsPbBr3 NRs exhibited ambipolar behaviors for up to two months after sample synthesis. A transition from ambipolar to p-type behaviors occurred after two months, and nearly all NRs completed the transition within two weeks. Moreover, template-grown CsPbBr3 NRs displayed better nanoscale electronic homogeneity compared to their conventional counterparts. The improved electronic uniformity and nanoscale homogeneity place the template-grown CsPbBr3 NRs in a unique advantageous position for optoelectronic applications.

Quantitative measurement of the charge carrier concentration using dielectric force microscopy

The charge carrier concentration profile is a critical factor that determines semiconducting material properties and device performance. Dielectric force microscopy (DFM) has been previously developed to map charge carrier concentrations with nanometer-scale spatial resolution. However, it is challenging to quantitatively obtain the charge carrier concentration, since the dielectric force is also affected by the mobility. Here, we quantitative measured the charge carrier concentration at the saturation mobility regime via the rectification effect-dependent gating ratio of DFM. By measuring a series of n-type GaAs and GaN thin films with mobility in the saturation regime, we confirmed the decreased DFM-measured gating ratio with increasing electron concentration. Combined with numerical simulation to calibrate the tip–sample geometry-induced systematic error, the quantitative correlation between the DFM-measured gating ratio and the electron concentration has been established, where the extracted electron concentration presents high accuracy in the range of 4 × 1016 – 1 × 1018 cm−3. We expect the quantitative DFM to find broad applications in characterizing the charge carrier transport properties of various semiconducting materials and devices.

Strained α-Sn thin films on highly lattice-mismatched Ge substrates

Strained α-Sn thin films and stanene (two-dimensional counterpart of α-Sn), which have attracted significant attentions due to their electronic properties as topological insulators, were mostly deposited on nearly lattice-matched substrates through molecular beam epitaxy. In this work, contrary to prior efforts, Sn thin films are deposited on highly lattice-mismatched Ge (100) substrates through physical vapor deposition of solid Sn sources. Three different methods, 1) Acetone/Isopropyl Alcohol (IPA), 2) Acetone/IPA/H2O(80°C), and 3) Hydrogen Peroxide (H2O2)/Hydrochloric Acid (HCl), are used to clean commercially available Ge (100) substrates. X-ray diffraction (XRD) characterizations indicate that compressively strained α-Sn thin films are formed on Ge substrates cleaned by Acetone/IPA/H2O(80°C) or H2O2/HCl, but no peak associated with crystalline α-Sn appears for thin films grown on Ge substrates cleaned by Acetone/IPA. Strained α-Sn thin films deposited on Ge substrates cleaned by Acetone/IPA/H2O(80°C) are further investigated using dielectric force microscopy (DFM). Although Ge substrates where α-Sn thin films are deposited are p-type, DFM images indicate that the α-Sn thin films are n-type. The α-Sn/Ge heterostructures that we demonstrate in this work might be promising in applications in advanced electronics and photonics.

Patterning GaSe by High-Powered Laser Beams.

We report the high-powered laser modification of the chemical, physical, and structural properties of the two-dimensional (2D) van der Waals material GaSe. Our results show that contrary to expectations and previous reports, GaSe at the periphery of a high-power laser beam does not entirely decompose into Se and Ga2O3. In contrast, we find unexpectedly that the Raman signal from GaSe gets amplified around regions where it was not expected to exist. Atomic force microscopy (AFM), dielectric force microscopy (DFM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) results show that laser irradiation induces the formation of nanoparticles. Our analyses demonstrate that, except for a fraction of Ga2Se3, these nanoparticles still belong to the GaSe phase but possess different electrical and optical properties. These changes are evidenced in the increased Raman intensity attributed to the near-resonance conditions with the Raman excitation laser. The elemental analysis of nanoparticles shows that the relative selenium content increased to as much as 70% from a 50:50 value in stoichiometric GaSe. This elemental change is related to the formation of the Ga2Se3 phase identified by Raman spectroscopy at some locations near the edge. Further, we exploit the localized high-power laser processing of GaSe to induce the formation of Ag–GaSe nanostructures by exposure to a solution of AgNO3. The selective reaction of AgNO3 with laser-irradiated GaSe gives rise to composite nanostructures that display photocatalytic activity originally absent in the pristine 2D material. The photocatalytic activity was investigated by the transformation of 4-nitrobenzenethiol to its amino and dimer forms detected in situ by Raman spectroscopy. This work improves the understanding of light–matter interaction in layered systems, offering an approach to the formation of laser-induced composites with added functionality.

Rapidly catalysis of oxygen evolution through sequential engineering of vertically layered FeNi structure

For practical hydrogen (H2) production via water electrolysis, large-scale design and fabrication of catalysts for high-efficiency electrochemical transformation of hydroxyl ions to oxygen in alkaline environment is of paramount importance to reduce energy losses. Using anodization, electroplating, and sequential electrodeposition, we herein realized a universal approach to vertically assemble 2D Fe-Ni hydroxide nanosheets on nickel foam surface as catalyst for oxygen evolution reaction (OER). The electrode exhibited OER onset overpotentials (η) of ~ 190mV and a low η of ~ 270mV at a current density of 100mAcm−2 in KOH electrolyte, making it one of the most active OER catalysts reported so far. The FeNi films deposited via sequential deposition are highly ordered, and electrode is stable even under long term or large current density operation. Based on the results of theoretical calculation, dielectric force microscopy (DFM) analysis, and capacitance measurements, we found that 2D Fe-Ni hydroxide-loaded electrode with enhanced conductivity facilitates the sufficiently conversion of active sites and ensures optimal adsorption energies for intermediates of OER, as a result of structural effects as well as introduction of optimal amount of Fe on surface.

Influences of water molecules on the electronic properties of atomically thin molybdenum disulfide

Although it is well known that the performances of two-dimensional transition metal dichalcogenide (2D-TMD) based devices are strongly affected by humidity, the roles of water molecules in the electronic properties of 2D-TMDs are still unclear. In this work, the influence of water molecules on the electrical properties of monolayer molybdenum disulfide (MoS2) is studied systemically using the dielectric force microscopy (DFM) technique. Taking the advantage of the DFM technique and other nondestructive characterization techniques, the electronic properties (surface potential, dielectrics, and carrier mobility) of atomically thin MoS2 exposed to different levels of humidity are investigated. Furthermore, Raman spectroscopy manifested the correlation between the optical phonon and the mobility drop of MoS2 flakes when subjected to humidity variations. Our results provide an in-depth understanding of the mechanism of water molecules interacting with MoS2.

Dielectric force microscopy: imaging charge carriers in nanomaterials without electrical contacts.

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.

Probe the Effects of Surface Adsorbates on ZnO Nanowire Conductivity using Dielectric Force Microscopy

Characterization of electric properties of nanomaterials usually involves fabricating field effect transistors (FET) and deriving materials properties from device performances. However, the quality of electrode contacts in FET devices heavily influences the device performance, which makes it difficult to obtain the intrinsic electric properties of nanomaterials. Dielectric force microscopy (DFM), a contactless method developed recently, can detect the low-frequency dielectric responses of nanomaterials without electric contact, which avoids the influence of electric contact and can be used to study the intrinsic conductivity of nanomaterials. Here we study the influences of surface adsorbates on the conductivity of ZnO nanowires (NWs) by using FET and DFM methods. The conductivity of ZnO NW is much larger in N2 atmosphere than that in ambient environment as measured by FET device, which is further proven by DFM measurement that the ZnO NW exhibits larger dielectric response in N2 environment, and the influence of electrode contacts on measurement can be ruled out. Based on these results, it can be concluded that the adsorbates on ZnO NW surface highly influence the conductivity of ZnO NW rather than the electrode contact. This work also verifies the capability of DFM in measuring electric properties of nanomaterials.

Contactless probing of the intrinsic carrier transport in single-walled carbon nanotubes

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.

Covalently Functionalized Metallic Single-Walled Carbon Nanotubes Studied Using Electrostatic Force Microscopy and Dielectric Force Microscopy

Contactless electrostatic force microscopy (EFM) and dielectric force microscopy (DFM) are demonstrated to be very powerful tools of characterizing the electronic properties of individual single-walled carbon nanotubes (SWCNTs). Taking the advantages of the tools, we confirm that the metallicity of metallic SWCNTs can be largely preserved upon dichlorocarbene functionalization ([2 + 1] cycloaddition) in comparison with the SWCNTs subject to the Prato reaction ([2 + 3] cycloaddition). This work demonstrates the distinct difference between sp2 rehybridized and sp3 rehybridized covalent configurations on their influences to electronic properties of metallic SWCNTs and supports the hypothesis that [2 + 1] cycloaddition could recover the sp2 hybridization on the sidewall of metallic SWCNTs and preserve the intrinsic electronic properties of SWCNTs.

Dielectric force microscopy: A new approach for observing charge behavior in nanomaterials

The behavior of charges transferring between the components of functional nanodevices would significantly impacts the physical/chemical process in the devices. Thus, comprehensively understanding the transfer behavior of electrons/ions in the operational devices is essential for optimizing the performance of functional nanodevices and exploring the application potential of nanomaterials. In general, the measurement results of field effect transistor (FET) present the statistic behavior of the carriers in the whole nanodevices, so the FET is powerless to observe the details of transfer process of charges. In addition, there are some drawbacks for the FET based on nanomaterials, such as difficult fabrication, complex electrode/nanomaterials contact and high costs. Therefore, we developed the dielectric force microscopy (DFM) to solve these problems. In this paper, we introduce the principle of DFM, and show some applications of interpreting how contactless DFM technique can quickly characterize the electrical properties of nanomaterials, such as one-dimension nanomaterials, nanoparticles and organic semiconducting film, with high spatial resolution. These applications demonstrate the capabilities of DFM for the characterization of the electrical properties of nanomaterials, and present the broad prospects of DFM in the research about the physical and chemical properties of nanomaterials and the performance of nanodevices.

Probing Electronic Doping of Single-Walled Carbon Nanotubes by Gaseous Ammonia with Dielectric Force Microscopy.

The electronic properties of single-walled carbon nanotubes (SWNTs) are sensitive to the gas molecules adsorbed on nanotube sidewalls. It is imperative to investigate the interaction between SWNTs and gas molecules in order to understand the mechanism of SWNT-based gas-sensing devices or the stability of individual SWNT-based field effect transistors (FETs). To avoid the Schottky barrier at the metal/SWNT contact, which dominates the performance of SWNT-based FETs, we utilize a contactless technique, dielectric force microscopy (DFM), to study the intrinsic interaction between SWNTs and gaseous ammonia molecules. Results show that gaseous ammonia affects the conductivity of semiconducting SWNTs but not metallic SWNTs. Semiconducting SWNTs, which are p-type doped in air, show suppressed hole concentration in ammonia gas and are even inverted to n-type doping in some cases.

Contactless Characterization of Electronic Properties of Nanomaterials Using Dielectric Force Microscopy

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.