H. J. Gao

Nanometer‐scale recording on an organic‐complex thin film with a scanning tunneling microscope

Nanometer‐scale recording on an organic‐complex thin film with a scanning tunneling microscope (STM) under ambient conditions is demonstrated. The recording marks are made by applying external voltage pulses between the tip and the highly ordered pyrolytic graphite substrate. A 30×30 nm2 STM image with recorded marks is given. The average recorded mark is 1.3 nm in diameter, which corresponds to a data storage density of about 1013 bits/cm2. The current–voltage characteristics measured by the STM show an insulator behavior for the unrecorded regions, and a conductor behavior for the recorded regions, which indicates that the data are recorded by local change of the electrical property of the films.

Fractal structure in the silver oxide thin film

Abstract The first two steps of the preparation of Ag–O–Cs photocathodes are the deposition of silver films and their oxidation with the glow discharge method. The fractal structure and texture structure in the silver oxide thin film have been characterized by transmission electron microscopy (TEM). The Hausdorf dimension D of the fractal structure is calculated to be 1.80±0.01. The formation mechanism of the fractal structure is also discussed.

Formation of ordered Ge quantum dots on the Si(111)–(7×7) surface

We present a pathway for the formation of ordered Ge quantum dots on Si(111)–(7×7) substrate. Self-assembled growth of Ge quantum dots on the Si(111)–(7×7) surface has been investigated using scanning tunneling microscopy. The Ge is grown on the substrate by solid phase epitaxy at room temperature. It has been found that the deposited submonolayer Ge can aggregate and form ordered Ge quantum dots on the surface through controlling the annealing temperature. The formation of ordered Ge quantum dots is due to the preferential adsorption sites of Ge on Si(111)–(7×7). The formed Ge quantum dots may have a great potential in the application of nanodevices.

Direct observation of a local structural transition for molecular recording with scanning tunneling microscopy

We present a direct observation of a structural transition at molecular scale in an organic p-nitrobenzonitrile (PNBN) thin film using scanning tunneling microscopy (STM). STM images clearly show an ordered molecular structure of unrecorded regions in the films, while the PNBN molecular arrangements are disordered in the recorded regions. The current–voltage (I–V) measurements from the STM demonstrate a great increase of the conductance transition in the recorded regions after the voltage pulses. Those results suggest that the recording mechanism may be due to a crystalline structural transition, which is consistent with our previously proposed mechanism in another organic system for ultrahigh density data storage.

Ionized‐cluster‐beam deposition and electrical bistability of C60–tetracyanoquinodimethane thin films

We report an ionized‐cluster‐beam (ICB) deposition and the electrical bistability of C60–tetracyanoquinodimethane (TCNQ) thin films. The films are fabricated by using an ionized‐cluster‐beam deposition method in a high vacuum system. The as‐deposited films were characterized by transmission electron microscopy and optical absorption spectroscopy, which verified the formation of the charge‐transfer complex system in C60–TCNQ thin films and the microstructure of these thin films. The structure and the electrical property of the ICB deposited Ag‐TCNQ thin films are also presented. The possible conductive mechanism of these ICB deposited thin films is discussed in the letter.

2D fractal pattern in fullerene doped polymer

Abstract Fractals in C60 doped tetracyanoquinodimethane thin films are reported. The fractal patterns and their microscopic features have been characterized by transmission electron microscopy (TEM). The results indicate that the observed fractals can be compared to the cluster-diffusion-limited-aggregation model. The fractal dimension has been determined to be 1.69±0.04. In addition, the formation mechanism of the fractals is discussed.

Near-infrared laser absorption spectroscopy of the CS+ cation

The (1, 0) band of the A(2)II-X(2)Sigma(+) transition of the CS+ cation has been measured in absorption at Doppler-limited resolution, between 12 400 and 13 000 cm(-1). The spectrum was recorded using differential velocity modulation detection and a Ti: sapphire laser as the near-infrared light source. The upsilon = 1 A(2)II upper state of the band is perturbed by the upsilon = 10 level of the ground state. A deperturbation analysis using > 300 rotational lines was carried out to derive accurate ground-and upper-state parameters, The ground-state rotational and fine-structure constants are in very good agreement with those from a Fourier transform emission study in which the ground-state constants were derived from combination differences

Self-assembled growth of ordered Ge nanoclusters on the Si(1 1 1)-(7×7) surface

The self-assembled growth of submonolayer Ge on the Si(111)-(7 x 7) surface grown by solid phase epitaxy has been studied using scanning tunneling microscopy. Ordered Ge nanoclusters on the surface are formed by the deposition of the submonolayer of Ge at room temperature and subsequent annealing. It is proposed that adsorbate Ge atoms are trapped in the attractive potential wells on the faulted half of the (7 x 7) unit cell

A new type of organometallic system for high density data storage by scanning tunneling microscopy

Abstract We report a new type of organometallic system, Ag-toluylene 2,4-dicarbamidonitrite (TDCN) composite thin film, for ultrahigh density data storage devices by scanning tunneling microscopy (STM). The Ag-TDCN thin film was prepared on a highly oriented pyrolytic graphite (HOPG) substrate using an ionized-cluster-beam (ICB) deposition method. Marks can be created on the thin film by employing a voltage pulse between the STM tip and surface of the HOPG substrate. The size of the created marks is about 50 nm and the corresponding data storage density is about 10 11 bits/cm 2 . The mechanism for the data storage can be attributed to the conductance transition in the complex system. This result shows a great potential for this new kind of organometallic composite system to find application in ultrahigh density data storage.

Patterns formed on the dimer vacancy array of Si(100) by self-assembly

The self-assembly of Si on the dimer vacancy array (DVA) is studied. Factors, such as the amount of deposition, substrate temperature and annealing temperature, which affect the self-assembly processes are discussed. The formation mechanism of the DVA is discussed.