Xian Ning Xie

Native oxide decomposition and local oxidation of 6H-SiC (0001) surface by atomic force microscopy

We have observed the native oxide decomposition and local oxide growth on 6H-silicon carbide (0001) surface induced by atomic force microscopy (AFM). When the biased AFM probe was scanned over surface areas, native oxide was decomposed and assembled into protruded lines. The decomposition is accompanied by simultaneous graphitization of the scanned areas, leading to metal–semiconductor contact as evidenced in I–V characteristics. When the probe was immobilized and longer bias duration applied, direct oxidation of silicon carbide (SiC) surface was achieved. The dielectrical properties of AFM oxide on SiC were also investigated in terms of interface barrier height.

Chemically Linked AuNP−Alkane Network for Enhanced Photoemission and Field Emission

Size and ligand effects are the basis for the novel properties and applications of metallic nanoparticles (NPs) in nanoelectronics, optoelectronics, and biotechnology. This work reports the first observation of enhanced photoelectron emission from metallic Au NPs ligated by alkanethiols. The enhancement is based on a conceptually new mechanism: the AuNP provides electrons while the alkane ligand emits electrons due to its low or negative electron affinity. Moreover, the AuNP-ligand chemical bonding is found to significantly facilitate the transmission of photoexcited electrons from the AuNP to the ligand emitter. Consequently the smooth NP film, which is a typical low-aspect-ratio two-dimensional structure, exhibits strong and stable field emission behavior under photoillumination conditions. The photoenhanced field emission is related to the interband and surface plasmon transitions in AuNPs, and a photoenhancement factor of up to approximately 300 is observed for the AuNP-based field emission. This is highly remarkable because field emission is often based on one-dimensional, high-aspect-ratio nanostructures (e.g., nanotubes and nanowires) with geometrical field enhancement effect. The chemical linkage of electron-supplying AuNP and electron-emitting alkane ligand represents a fundamentally new mechanism for efficient photoexcitation and emission. Being low-temperature/solution processable, and inkjet printable, AuNPs may be a flexible material system for optoelectronic applications such as photodetection and photoenhanced field emission.

Spatially resolved diagnosis of stress-induced breakdown in oxide dots by in situ conducting atomic force microscopy

We report an investigation on the stress-induced breakdown (BD) in ultrathin oxide grown by atomic force microscopy (AFM oxide). A conducting atomic force microscopy (c-AFM) technique was employed to stress the AFM oxide and examine its BD behavior. It was found that thermal annealing has a strong impact on the dielectric strength of AFM oxide. The stress-induced trap generation probability, Pt, could be reduced by ∼50% after annealing the oxide at elevated temperatures. Such a thermal effect is related to the local structural relaxation and trap state minimization in AFM oxide upon annealing. The spatially resolved current images allow a microscopic diagnosis of the distribution of BD sites: isolated single BD spots and laterally propagated BD areas were observed in an oxide dot. Soft and hard breakdown sites were also distinguished on the current images.

Polymeric conical structure formation by probe-induced electrohydrodynamical nanofluidic motion

We report the creation of polymeric structures by atomic force microscopy (AFM) probe induced electrohydrodynamic (EHD) instability and nanofluidic flow. By biasing the AFM probe in a high field regime, single conical structure was produced on poly(methylmethacrylate) due to the initiation of strong EHD instability in the locally heated polymer melts. The pattern formation is dominated by the interplay of polymer EHD motion, polymer ablation, and AFM tip repulsion. The dependence of cone formation probability on the bending of AFM cantilevers with different stiffness was also discussed.

Negative differential resistance based on electron injection/extraction in conducting organic films

This work reports a mechanism of negative differential resistance (NDR) observed for perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) films. The NDR is based on electron injection and extraction at the metal/PTCDA interface, and is governed by the joint effect of electronic and ionic components. Consequently, the NDR behavior exhibits a monotonous dependence on the voltage scan rate, and the number of NDR peaks is also sensitive to the work function of metal electrodes. The results provide further understanding on the diverse manifestation of NDR, and would be useful in organic electronic applications.

Observation of a 6×6 superstructure on 6H–SiC (0001) by reflection high energy electron diffraction

A 6×6 reconstruction consisting of a silicon-rich phase has been observed on 6H–SiC (0001) after annealing a silicon-enriched 3×3 6H–SiC (0001) surface at 900 °C. Our results show that the 6×6 reconstruction obtained is a long-range reconstruction and not a psuedoperiodic structure due to incommensurately adsorbed graphite honeycombs as suggested previously. We propose that the 6×6 structure arises from the absence of consecutive tetrahedral clusters in a tetrahedral cluster array with 3×3 periodicity. Annealing the 6×6 structure further at 1200 °C results in a carbon-rich 6√3×6√3R30 structure.

UV-visible-near infrared photoabsorption and photodetection using close-packed metallic gold nanoparticle network

Photocurrent generation and photodetection are usually based on semiconductor crystals including Si, CdS, and PbS. This work reports the enhanced photoabsorption and photodetection of close-packed metallic Au nanoparticles (NPs) in the UV-VIS (visible)-NIR (near infrared) region. Photoabsorption in the UV-VIS regions is associated with the interband transition and surface plasmon resonance of AuNPs, while the enhanced NIR absorption is due to the collective effect of interacting AuNPs in the close-packed network. Consequently, the AuNPs exhibits photodetection behavior in the wavelength range of 300–1500 nm. It is proposed that the inter-AuNP photoejection and delocalization of electron-hole pairs changes the carrier lifetime and transit dynamics in favor of photocarrier conduction, thus significantly facilitating photocurrent generation in the metallic AuNP close-pack. Moreover, due to the power-law conduction mechanism in AuNP networks, the quantum yield of AuNPs can be tuned from 10−6 to 10−1 photoelec...

Distinguishing the H3 and T4 silicon adatom model on 6H–SiC(0001) √3×√3R30° reconstruction by dynamic rocking beam approach

Silicon adatoms can occupy either the H3 or T4 site, corresponding to the hollow or on-top site of the hexagonal unit cell of the 6H–SiC(0001)−√3×√3R30° superstructure. Distinguishing these two possibilities is impossible with the one-beam calculation method in surface electron diffraction. We provide the experimental evidence to differentiate between these two possibilities using a dynamic, multiple rocking beam approach and demonstrate the sensitivity of this approach to the lateral displacement of atoms on the surface. Our study shows that the rocking curve based on the T4 model provides a more convincing fit to the experiment compared with the H3 model, with a metric distance as low as 7%. We also identify A-type termination to be the most likely bulk-truncated substrate face among the three possible truncated faces for the 6H–SiC polytype. Coverage dependence of the silicon adatoms on the profile of the rocking curve is also investigated.

Deuterium-oxygen exchange on diamond (100)—a study by ERDA, RBS and TOF-SIMS

Abstract The exchange of radio-frequency plasma-excited atomic O with chemisorbed D, and vice versa, on single crystalline diamond (100) 2×1 has been investigated by elastic recoil detection analysis (ERDA), Rutherford backscattering spectrometry (RBS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). It was found that the O-D, as well as D-O, exchange processes were thermally activated by the diamond substrate. The surface D was only partially exchanged by atomic O at room temperature. At elevated temperatures, the replacement of D by O was greatly enhanced, with 80% substitution by O at 300 °C. It was also observed that the uptake of O proceeded more readily on the pre-deuterated surface compared to the clean surface. Ultra-shallow depth profiling revealed that atomic beam treatment of single crystalline samples at 800 °C resulted in only superficial uptake of D and O, with no surface incorporation within the shallow analysis depth. The replacement of pre-adsorbed O by RF-excited atomic D was also studied by TOF-SIMS. Pre-adsorbed O was relatively stable and resisted removal at 300 °C. D dosed onto the oxygenated surface was found to co-adsorb with O, possibly as surface bound OD species. Mechanisms for the O-D and D-O exchange processes were discussed in connection with the atomic structure of the C(100) surface.

A percolating membrane with superior polarization and power retention for rechargeable energy storage.

Polarization is associated with the property of a material to form electric dipoles in the direction of an external field E. In this case, the separated positive and negative charges remain in the material, and thus create an internal field. The amount of polarizable charges determines the value of polarization P in the unit of C·cm−2. When the external field is removed, the material can maintain its polarized charges for certain time, and this ability is defined to be the polarization or charge retention. Basically, there are two types of polarizable materials – dielectrics and ferroelectrics. Dielectrics are electrically insulating, and their polarization is based on the slight shift of charges under a field.[1–6] Ferroelectric polarization is related to field-induced phase transitions which involve the switching of ionic domain walls.[7–10] Dielectrics are often used as insulators and frequency resonators,[1–6] while ferroelectrics are applicable to switching memory devices.[7–10] Due to their small polarization P in the order of μC·cm−2, neither dielectrics nor ferroelectrics are suitable for electric energy storage. This is because the amount of polarizable charges is so small that the corresponding current is only μA·cm−2, which is too low a current compared to that of common energy-storage devices such as batteries and supercapacitors.[11–15] This work reports the discovery of a third type of polarizable material – the percolating organic membrane. Compared to dielectrics and ferroelectrics, this new class of polarizable membrane exhibits a few extraordinary properties. First, it has the largest polarization known so far, with P (up to 1.0 C·cm−2) being 105–106 times that of dielectrics and ferroelectrics. Second, the membrane exhibits good polarization retention, and is able to retain 50% of the polarized charges for 24 hours. In addition, the membrane polarization follows an anti-intuition mechanism in which percolation effect is at work, making thicker membranes much more polarizable and ionic-conducting than thinner ones. The large polarization and long charge retention renders the membrane a promising material in energy storage applications. In the following sections, we present the above unique features of the membrane, and demonstrate its immediate applicability in storing the electric energy generated by a Si solar panel.