Noncontact Scanning Nonlinear Dielectric Microscopy Research Articles

Experimental study of electric dipoles on an oxygen-adsorbed Si(100)-2 × 1 surface by non-contact scanning nonlinear dielectric microscopy

Oxygen-adsorption on a Si(100)-2 × 1 surface is investigated by using non-contact scanning nonlinear dielectric microscopy (NC-SNDM). On the Si(100)-2 × 1 surface exposed to oxygen (O2) gas at room temperature, several variations in atomic configuration and electric dipole moment of dimers are observed. Models are proposed for oxygen adsorption which are consistent with the topographies and electric dipole moment distributions obtained by NC-SNDM.

Atomic-dipole-moment induced local surface potential on Si(111)-(7 × 7) surface studied by non-contact scanning nonlinear dielectric microscopy

We have performed the site-specific, quantitative measurement of a local surface potential induced by atomic dipoles on a Si(111)-(7 × 7) surface by non-contact scanning nonlinear dielectric microscopy (NC-SNDM) combined with an atom-tracking technique. The measured potentials were quantitatively consistent with those estimated by a simultaneous measurement of the tunneling current, which validates a previously proposed hypothetical mechanism that explains the unexpected resemblance between the dipole and time-averaged tunneling current images in NC-SNDM imaging. The results show that an asymmetry arising in the current-voltage characteristics within the tunneling regime is governed by the local surface potential induced by atomic dipoles.

Improved study of electric dipoles on the Si(100)-2 × 1 surface by non-contact scanning nonlinear dielectric microscopy

We studied a Si(100)-2 × 1 surface by non-contact scanning nonlinear dielectric microscopy (NC-SNDM). Simultaneously taken images of the topography and electric dipole moment distribution show that negative electric dipole moments are locally formed on individual dimers on the surface. In addition, we obtained the dc bias voltage dependence of the εlocal(3) signal on a specific dimer by using an atom-tracking technique with NC-SNDM. We observed that the electric dipole induced a surface potential of around −250 mV on the dimer.

Atomic dipole moment distribution on a hydrogen-adsorbed Si(111)-(7 × 7) surface observed by noncontact scanning nonlinear dielectric microscopy

Noncontact scanning nonlinear dielectric microscopy (NC-SNDM) can atomically resolve the polarization distribution on material surfaces. We observed a hydrogen-adsorbed Si(111)–(7 × 7) surface using NC-SNDM and found that hydrogen-adsorbed Si adatoms had lower dipole moments than bare Si adatoms. We discuss the charge distribution around a hydrogen-adsorbed Si adatom in terms of its geometry and the electronegativities of hydrogen and silicon. Our model suggests that the charge distribution around a hydrogen-adsorbed adatom is nearly symmetric and it qualitatively explains the experimental results. We thus conclude that the hydrogen-adsorbed Si adatoms are both electrically and chemically passivated.

Simultaneous measurement of tunneling current and atomic dipole moment on Si(111)-(7 × 7) surface by noncontact scanning nonlinear dielectric microscopy

Non-contact scanning nonlinear dielectric microscopy (NC-SNDM) can resolve the topography and dipole moment distribution of a Si(111)-(7 × 7) surface on an atomic level. We discuss the origin of the atomic contrast in dipole moment images based on simultaneously acquired time-averaged tunneling current images. The dipole moment images are found to have the same characteristics as the simultaneously acquired current images. This similarity between these two images does not originate from circuit crosstalk since it is suppressed by our circuit design that decouples the tip-sample capacitance and the tunneling current. Constant-height images also indicate that the atomic contrast is not artificially caused by feedback crosstalk. These results suggest that the atomic contrast is due to the variation in the tip-sample capacitance caused by modulation of atomic dipole moments by the local density of states of the surface.

Atomic Scale Imaging of TiO2(100) Reconstructed Surfaces by Noncontact Scanning Nonlinear Dielectric Microscopy

We performed noncontact scanning nonlinear dielectric microscopy (NC-SNDM) measurements to investigate both the topography and polarization distribution images of a reconstructed TiO2(100) surface. To acquire an atomic resolution image of the (110) surface, which is the most stable in TiO2, NC-SNDM requires annealing at a temperature higher than that for scanning tunneling microscopy (STM). However, in the case of the (100) surface, we found that annealing at the same temperature as that used for STM was sufficient. The two types of reconstructed surfaces were measured using NC-SNDM. We confirmed we could even achieve NC-SNDM measurements in a dielectric material by reducing the thickness of the specimen and making the specimen nonstoichiometric without any annealing. These results show that NC-SNDM has superb capabilities that STM does not have, including the capability to measure the topography and polarization distribution even in a dielectric material, by inducing polarization near the surface without any means of inducing conductivity.

Atomic Scale Imaging of TiO2(100) Reconstructed Surfaces by Noncontact Scanning Nonlinear Dielectric Microscopy

We performed noncontact scanning nonlinear dielectric microscopy (NC-SNDM) measurements to investigate both the topography and polarization distribution images of a reconstructed TiO2(100) surface. To acquire an atomic resolution image of the (110) surface, which is the most stable in TiO2, NC-SNDM requires annealing at a temperature higher than that for scanning tunneling microscopy (STM). However, in the case of the (100) surface, we found that annealing at the same temperature as that used for STM was sufficient. The two types of reconstructed surfaces were measured using NC-SNDM. We confirmed we could even achieve NC-SNDM measurements in a dielectric material by reducing the thickness of the specimen and making the specimen nonstoichiometric without any annealing. These results show that NC-SNDM has superb capabilities that STM does not have, including the capability to measure the topography and polarization distribution even in a dielectric material, by inducing polarization near the surface without any means of inducing conductivity.

New evaluation of fullerene molecules on Si(111)-(7 × 7) reconstructed structure using non-contact scanning non-linear dielectric microscopy

Fullerene (C60) molecules on an Si(111)-(7×7) surface have been investigated using non-contact scanning non-linear dielectric microscopy (NC-SNDM) under an ultra-high vacuum. The topography, the interface between the C60 molecule and Si adatoms, and the internal structure of the C60 molecules were successfully investigated. For ∼0 ML and ∼0.4 ML coverage, both phase reversal sites and sites without phase reversal could be observed in the first order phase (θ1) image. On the other hand, for 1 ML coverage, phase reversal could not be identified. These results indicate that charge transfer only occurred from Si adatoms to C60 molecules at three-fold symmetric sites on the Si(111)-(7×7) surface, and the electric dipole moment is reflected in the electronic state of the C60 molecules. The internal structure of C60 molecules was clearly observed in topography by the second order amplitude (A2) feedback signal for 1 ML coverage, reflecting the LDOS originating from the t1u orbital.

Observation of Polarization Distribution on Si(111) Surface by Scanning Nonlinear Dielectric Microscopy

Noncontact scanning nonlinear dielectric microscopy (NC-SNDM) has the ability to simultaneously acquire the surface topography and polarization distribution at the nanoscale through the measurement of local nonlinear dielectric constants of materials. NC-SNDM was here applied to the observation of Si(111) reconstructed surfaces. Images of the polarization distribution clearly distinguished disordered regions of the surface, often called 1×1 regions, at the boundaries between the regular (7×7) domains. We acquired polarization images of surfaces with different sizes of 1×1 regions and show that NC-SNDM has the potential to image trapped charges of surfaces and interfaces.

Observation of Polarization Distribution on Si(111) Surface by Scanning Nonlinear Dielectric Microscopy

Noncontact scanning nonlinear dielectric microscopy (NC-SNDM) has the ability to simultaneously acquire the surface topography and polarization distribution at the nanoscale through the measurement of local nonlinear dielectric constants of materials. NC-SNDM was here applied to the observation of Si(111) reconstructed surfaces. Images of the polarization distribution clearly distinguished disordered regions of the surface, often called 1×1 regions, at the boundaries between the regular (7×7) domains. We acquired polarization images of surfaces with different sizes of 1×1 regions and show that NC-SNDM has the potential to image trapped charges of surfaces and interfaces.

Scanning nonlinear dielectric microscopy

Abstract

Observation of electrochemical capacitance in a graphite surface by noncontact scanning nonlinear dielectric microscopy

We have developed a noncontact, scanning nonlinear dielectric microscopy (NC-SNDM) system operated under ultrahigh vacuum and have used it to observe the surface of graphite. By using the NC-SNDM technique ($2\ensuremath{\omega}$ amplitude feedback mode), we succeeded in obtaining clear SNDM images of the graphite surface. In the SNDM image patterns (inverted contrast $\ensuremath{\omega}$ amplitude image), a number of convex spots, with threefold symmetry, positioned at the corners of hexagons were observed when the probe tip was near the graphite surface. In contrast, a number of convex spots, with threefold symmetry, were also observed in the normal contrast $\ensuremath{\omega}$ amplitude image when the distance between the probe tip and graphite was large. Current images originating from tunneling were also observed in NC-SNDM and were similar to the $\ensuremath{\omega}$ amplitude images. Electrochemical capacitance between the probe tip and graphite surface with tunneling was introduced to investigate the origin of the SNDM signal. Applying this model to the NC-SNDM measurement, we found that the $\ensuremath{\omega}$ amplitude signal is dependent on the ratio of the slope of the local density of states (LDOS) and the energy curve, and the LDOS at the Fermi energy in the graphite surface. The convex spots and hollow spots in the hexagons on the graphite surface also originate from the LDOS in the NC-SNDM measurements. This means that the SNDM signal for the graphite surface reflects the LDOS on the graphite surface. Considering the relationship between the $\ensuremath{\omega}$ amplitude signal and LDOS on the graphite surface, the current signal observed in the NC-SNDM measurement originates from the tunneling effect between the probe tip and graphite surface. The proposed technique can potentially detect not only a unique carbon surface but also organic molecules on a metallic surface.

Imaging of the surface structure of TiO2(110) by noncontact scanning nonlinear dielectric microscopy

We have demonstrated that atom-resolved imaging of the n-type wide-gap semiconductor TiO2(110) can be achieved by noncontact scanning nonlinear dielectric microscopy (NC-SNDM). In the NC-SNDM images, parallel bright stripes along the [001] direction, reflecting Ti4+ rows, could be observed. An upward dipole moment was simultaneously measured at each Ti4+ site, although downward polarization could not be detected at the O2− sites. These results suggest that both the topography [εlocal(4)] and the electrical dipole moment [εlocal(3)] are influenced by the Ti4+ interaction between the tip and sites.

Noncontact scanning nonlinear dielectric microscopy imaging of TiO2(110) surfaces

The authors simultaneously observed the typical topography and local electric dipole moment distribution of Ti atoms on a TiO2(110) 1×1 structure using noncontact scanning nonlinear dielectric microscopy (NC-SNDM). The upward polarization observed in polarization distribution images implies that the topography determined from the higher-order nonlinear dielectric constant signals is that of Ti4+. They also have examined the difference between the images formed by NC-SNDM and scanning tunneling microscopy. To acquire atomic resolution images using NC-SNDM, it was found that higher annealing temperatures were required.

Investigation of interface between fullerene molecule and Si(111)-7×7 surface by noncontact scanning nonlinear dielectric microscopy

Fullerene (C60) molecules deposited on Si(111)-7×7 reconstructed (7×7) surface were investigated by noncontact scanning nonlinear dielectric microscopy (NC-SNDM) under ultrahigh vacuum conditions. Both topography and induced electric dipole moment of individual C60 molecules were successfully resolved with molecular-scale resolution. Charge transfer from the Si dangling bonds to the C60 molecule occurs and induces an electric dipole moment at almost all C60 sites. On the application of an alternating voltage, the downward electric dipole moment is nonlinearly modulated due to the electric property at the interface. The NC-SNDM technique can thus detect the electric dipole moment induced at interface between the C60 molecule and the Si adatom in 7×7 surface by the charge transfer from the Si adatom to C60 molecule.

Observation of local dipole moment of Si atoms on Si(100) surfaces using noncontact scanning nonlinear dielectric microscopy

To confirm the performance of noncontact scanning nonlinear dielectric microscopy (NC-SNDM), we attempted to determine the local dipole moment of Si atoms on a cleaned Si (100) surface under UHV conditions. From the topography images, atomically flat terraces, step structures, and defects were clearly recognized, and paired bright spots with a 2×1 symmetry were observed with clear contrast. In addition, we observed the local electric dipole moment distribution of Si atoms on a 2×1 structure. This revealed that the surface is naturally biased with an offset potential of −0.2V, and the direction of the local dipole moment is upward at dimer sites for bias values above −0.2V.

Observation of the Si(111)-7×7 Reconstructed Surface and Fullerene Molecule by the Non-contact Scanning Nonlinear Dielectric Microscopy

The Si(111)-7×7 reconstructed surface and fullerene (C60) molecules deposited on Si(111)-7×7 reconstructed surface were investigated by using a new technique called non-contact scanning nonlinear dielectric microscopy (NC-SNDM) under ultra-high vacuum conditions. A local atomic electric dipole moment distribution of Si atoms on Si(111)-7×7 reconstructed surface was resolved. This is the first successful demonstration of direct atomic dipole moment observation achieved in the field of capacitance measurement. Furthermore, both topography and induced electric dipole moment of individual C60 molecules as well as internal structure of the C60 molecule were successfully resolved with atomic-scale resolution. The NC-SNDM technique can yield the internal structure of the C60 molecule and the position where the charge transfer of the C60 molecules occurs on the Si(111)-7×7 reconstructed surface.

Atomic Dipole Moment Distribution of Si Atoms on aSi(111)−(7×7)Surface Studied Using Noncontact Scanning Nonlinear Dielectric Microscopy

A local atomic electric dipole moment distribution of Si atoms on Si(111)-(7 x 7) surface is clearly resolved by using a new technique called noncontact scanning nonlinear dielectric microscopy. The dc-bias voltage dependence of the atomic dipole moment on the Si(111)-(7 x 7) surface is measured. At the weak applied voltage of -0.5 V, a positive dipole moment is detected on the Si adatom sites, whereas a negative dipole moment is observed at the interstitial sites of inter Si adatoms. Moreover, the quantitative dependence of the surface dipole moment as a function of the applied dc voltage is also revealed at a fixed point above the sample surface. This is the first successful demonstration of direct atomic dipole moment observation achieved in the field of capacitance measurement.

Observation of the Si(111)7 × 7 atomic structure using non-contact scanning nonlinear dielectric microscopy

Non-contact scanning nonlinear dielectric microscopy (NC-SNDM) operated under ultrahigh vacuum (UHV) conditions was developed. This microscopy enables the simultaneousmeasurement of the topography and dielectric properties of a specimen. For electricallyconductive materials, the tunnelling current is also measurable. The atomic structure ofSi(111)7 × 7 was successfully resolved using this new SNDM technique. This is the first report on theachievement of atomic resolution in capacitance measurements.

Non-Contact Scanning Nonlinear Dielectric Microscopy Utilizing Higher Order Nonlinear Dielectric Signal

The contact-type scanning nonlinear dielectric microscopy (SNDM) can measure the distribution of linear and nonlinear dielectric properties of insulators with sub-nanometer resolution. SNDM, however, has tip-abrasion problem and this problem is one of the obstacles for the high quality image. In this paper, in order to improve SNDM, we propose a non-contact scanning nonlinear dielectric microscopy (NC-SNDM) which focused on the nonlinear dielectric signal for controlling the non-contact condition. With NC-SNDM technique, topographic and SNDM images were obtained successfully.