STM-induced photon emission spectroscopy of granular gold surfaces in air

Light emission has been detected under ambient conditions in the tip–sample region of a scanning tunneling microscope (STM) consisting of an etched gold tip and a granular gold film. The photon yield as a function of surface geometry (photon mapping) has been studied. By means of STM, it was possible to measure photon emission spectra locally. We have studied the effect of grain size and applied bias voltage on the spectrum. We found that the peak position in the photon emission spectrum shifts to a shorter wavelength when increasing the bias voltage and shifts to a longer wavelength when tunneling to larger grains. These effects can be understood in a simple model which considers tunneling electrons exciting localized surface plasmons which decay by emitting photons.     

Manipulating Ge quantum dots on ultrathin SixGe1−x oxide films using scanning tunneling microscope tips

Germanium quantum dots (QDs) were extracted from ultrathin Si_xGe_1−x oxide films using scanning tunneling microscope (STM) tips. The extraction was most efficiently performed at a positive sample bias voltage of +5.0 V. The tunneling current dependence of the extraction efficiency was explained by the electric field evaporation transfer mechanism for positive Ge ions from QDs to STM tips. Ge QDs (∼7 nm) were formed and isolated spatially by extracting the surrounding Ge QDs with an ultrahigh density of >10¹² cm⁻². Scanning tunneling spectroscopy of the spatially-isolated QDs revealed that QDs with an ultrahigh density are electrically-isolated from the adjacent dots.     

Scanning tunneling microscopy with atomic resolution on ReS2 single crystals grown by vapor phase transport

Atomic resolution images of layered transition metal-dichalcogenide ReS₂ single-crystals (n-type semiconductor) were obtained using a scanning tunneling microscope with a positive tip. In most cases only unresolved clusters of four rhenium atoms could be seen. Occasional images with higher resolution showed that these bright structures consist of four separated atoms. The symmetry of the imaged atoms is identical to that of the rhenium sublattice but not to that of the sulfur atoms. We conclude therefore that the main contribution to the tunneling current is due to the rhenium atoms, although the sulfur atoms are placed by about 0.15 nm closer to the tip. Thus for our positive bias of the tip the tunneling electrons originate from occupied rhenium states in the valence band of the semiconductor.     

Chemisorption of atomic oxygen on Pd(1 1 1) studied by STM

Atomic oxygen resulting from the dissociation of O₂ on Pd(1 1 1) at low coverage was studied in a variable temperature scanning tunneling microscope (STM) in the range from 30 to 210 K. Oxygen atoms, which typically appear as 30-40 pm deep depressions on Pd(1 1 1), occupy fcc hollow sites and form ordered p(2 × 2) islands upon annealing above 180 K. The mobility of the atoms diminishes rapidly below 180 K, with an approximate diffusion barrier of 0.4-0.5 eV. Oxygen atom pairs produced by thermal dissociation of O₂ at 160 K occupy both fcc and hcp hollow sites. The atoms travel approximately 0.25 nm after dissociation, and the distribution of pairs is strongly influenced by the presence of subsurface impurities within the Pd sample. At much lower temperatures, the STM tip can dissociate oxygen molecules. Dissociation occurs at sample bias voltages exceeding approximately 0.1 V. Following tip-induced dissociation, the product atoms occupy only fcc hollow sites. Oxygen atoms can be manipulated via short range repulsive interactions with the STM tip.     

Enhanced photon emission by field emission resonances and local surface plasmon in tunneling junction

We investigated the photon emission spectra on Ag (111) surface excited by tunneling electrons using a low temperature scanning tunneling microscope in ultrahigh vacuum. Characteristic plasmon modes were illustrated as a function of the bias voltage. The one electron excitation process was revealed by the linear relationship between the luminescence intensity and the tunneling current. Luminescence enhancement is observed in the tunneling regime for the relatively high bias voltages, as well as at the field emission resonance with bias voltage increased up to 9 V. Presence of a silver (Ag) nanoparticle in the tunneling junction results in an abnormally strong photon emission at the high field emission resonances, which is explained by the further enhancement due to coupling between the localized surface plasmon and the vacuum. The results are of potential value for applications where ultimate enhancement of photon emission is desired.     

Several large-scale superperiodicities on highly oriented pyrolytic graphite observed by scanning tunneling microscopy

On freshly cleaved highly oriented pyrolytic graphite we observed large-scale superperiodicities by a scanning tunneling microscope at room temperature in air. Several hexagonal superstructures with periods of 30 nm, 4.2 nm, 2.4 nm, and 2.0 nm, respectively, and a strip-like superstructure with a period of 1 nm were obtained. With exception of the largest hexagonal superperiodicity (30 nm spacing), all other superstructures are superimposed on the atomic corrugation of graphite. The origin of these superstructures is not clear yet. We assume that they arise from crystal defects in graphite. The hexagonal superstructure may be caused by the Moiré effect due to the rotational misorientation of the two top layers or of two successive layers near the surface.     

Donor and acceptor-like electronic states in a one-dimensional semiconductor

Donor and acceptor-like electronic states were observed near the conduction and valence band, respectively, in a semiconducting single-wall carbon nanotube using scanning tunneling microscopy and spectroscopy. They are observed to be, spatially, at the same location and spread within a few nm. Their physical origin is suggested as locally formed excessive and deficient number of electrons per a closed carbon network.     

Effect of bias voltage and electrodes size on the Pd conductance histograms

Conductance histograms of palladium nanocontacts in ultra high vacuum (UHV) were experimentally studied at room temperature using scanning tunneling microscope (STM). Our results show that the resolution of the pure Pd peaks, at 1.8 and around 3G₀, in the histograms, depends on the bias voltage and the electrodes size. The size of the electrodes should be as small as possible to achieve the higher extraction of hydrogen from them and therefore preventing the diffusion of H from the bulk to the nanocontacts during the conductance measurements, particularly at low bias voltage. This could explain why peaks have not been observed previously in the Pd histograms, using STM techniques in UHV.     

Nanoscale structuring of SrRuO3 thin film surfaces by scanning tunneling microscopy

Surface modifications through line etching of SrRuO₃ thin films have been carried out using a scanning tunneling microscope under ambient conditions. The line etching is found to be dependent on both bias voltage and scan speed for a given number of scan repetitions. We observe that an applied voltage above a threshold value is required for successful line etching. The depth of the etched lines is increasing with increasing bias voltage and scan repetitions as well as with decreasing scan speed. Moreover, sub-50 nm laterally confined mesa structures could be reproducibly etched on the SrRuO₃ thin film surfaces.     

White light-emitting quantum dot diodes and tuning of luminescence processes

A novel white light-emitting diode based on a large Stokes shift (~200 nm) and using pure green light-emitting CdSeS quantum dots (QDs) with an Ag/ZnSnO/QDs/spiro-TPD/ITO structure has been fabricated in which ZnSnO and spiro-TPD are served as the electron and hole transport layer, respectively. The large Stokes shift of the CdSeS QDs excludes potentially Förster resonance energy transfer process, which allows spiro-TPD to act as both an emitter and hole transport layer. The devices exhibit a wide EL spectrum consisting of three components: blue emission from spiro-TPD, green emission from QD band–band recombination, and red emission from QD surface-state recombination. We further found that as the intensity ratios among these three components vary with bias the color of the QD light-emitting diodes is tunable. The device displays a good white light-emitting characteristic with CIE coordinates of (0.281, 0.384) at an appropriate bias.     

Effect of annealing treatment on luminescence property of MgO nanowires

Magnesium oxide nanowires (NWs) were synthesized by a current heating process. The effect of annealing treatment in air on their structural and optical characteristics was investigated. As-grown NWs in diameter ranging from 30 to 70 nm and length up to several 10 μm were obtained. Carbon-coating layers were observed in the as-grown NWs which were oxidized during the annealing treatment at high temperature. Ionoluminescence spectra of the as-grown and annealed NWs have showed two emission peaks centered at 360 nm (UV emission) and 492 nm (green emission). The intensities of green emission were maximum at the annealing temperature of 650 °C for 2 h, whereas those of UV emission were decreased with increasing the annealing temperature. It is anticipated that maximum green emission correlates to the sufficient density of oxygen vacancies which was occurred by the optimum annealing parameters of both temperature and time.     

Effect of substrate doping concentration on quantum well states of Pb island grown on Si(1 1 1)

The interaction between the metallic film/island and the semiconductor substrate is important to the electronic properties of metallic nanostructure grown on semiconductor substrate. Here, we report a series of comparison experiments to investigate the effect of doping concentration of Si substrates on the quantum well state (QWS). Using scanning tunneling microscopy, we observed that the apparent QWS energy positions show a strong dependence on the substrate used and on the sample temperature. Further experimental results by varying the height of scanning tunneling microscope tip over the Pb island uncovered that the observed apparent QWS energy position changes mainly come from the partial bias voltage drop on the combined resistance of the Pb wetting layer and the substrate, which is comparable with the vacuum tunneling resistance at low temperatures.     

隧道电子激发的辛硫醇条纹相自组装膜上卟啉单分子荧光

利用扫描隧道显微镜诱导发光技术,对单个卟啉分子的电致荧光现象进行了研究. 为了避免金属衬底对单个卟啉分子的荧光淬灭,利用条纹状辛硫醇自组装膜作为脱耦合层,实现了单个中性卟啉分子的电致荧光,并且发现分子荧光的产生呈现双极性特征. 另外,分子瓣上的荧光强度要强于分子中心的荧光强度.     

Characterization of heat-assisted magnetic probe recording on CoNi/Pt multilayers

A method of heat-assisted magnetic recording (HAMR) potentially suitable for probe-based storage systems is characterized. In this work, field emission current from a scanning tunneling microscope (STM) tip is used as the heating source. Pulse voltages of 2–7 V were applied to a CoNi/Pt multilayered film fabricated on either bare silicon or oxidized silicon substrates. Different types of Ir/Pt and W STM tips were used in the experiment. The results show that thermally recorded magnetic marks are formed with a nearly uniform mark size of 170 nm on the film fabricated on bare silicon substrate when the pulse voltage is above a threshold voltage. The mark size becomes 260 nm when they are written on the identical film fabricated on an oxidized silicon substrate. The threshold voltage depends on the material work function of the tip, with W having a threshold voltage about 1 V lower than Pt. A synthesized model, which contains the calculation of the emission current, the simulation of heat transfer during heating, and the study of magnetic domain formation, was introduced to explain experimental results. The simulation agrees well with the experiments.     

Scanning probe microscopy study of exfoliated oxidized graphene sheets

Exfoliated oxidized graphene (OG) sheets, suspended in an aqueous solution, were deposited on freshly cleaved HOPG and studied by ambient AFM and UHV STM. The AFM images revealed oxidized graphene sheets with a lateral dimension of 5–10 μm. The oxidized graphene sheets exhibited different thicknesses and were found to conformally coat the HOPG substrate. Wrinkles and folds induced by the deposition process were clearly observed. Phase imaging and lateral force microscopy showed distinct contrast between the oxidized graphene and the underlying HOPG substrate. The UHV STM studies of oxidized graphene revealed atomic scale periodicity showing a (0.273 ± 0.008) nm × (0.406 ± 0.013) nm unit cell over distances spanning few nanometers. This periodicity is identified with oxygen atoms bound to the oxidized graphene sheet. I(V) data were taken from oxidized graphene sheets and compared to similar data obtained from bulk HOPG. The dI/dV data from oxidized graphene reveals a reduction in the local density of states for bias voltages in the range of ±0.1 V.     

Color variety of up-conversion emission of Er3+/Yb3+ co-doped phosphate glass ceramics

The sample of Er³⁺/Yb³⁺ co-doped phosphate glass ceramic was prepared. At 975 nm laser diode (LD) excitation, the strong up-conversion (UC) emissions were observed, which were the UC green emission at 510–570 nm and the UC red emission at 636–692 nm, respectively. At low pump power (126 mW), the red emission is primary, and the color purity R_cp is 0.81. With the increasing of pump power, the emission color gradually varies from red to green. The intensity of the green emission is stronger compared to that of the red emission at high power (868 mW), and the color purity R_cp is 0.76. Thus, this material can be applied to fluorescence anti-counterfeiting by the color variety of UC emission under different pump power.     

Size-dependent single electron tunneling effect in Au nanoparticles

We investigated single electron tunneling (SET) behavior of dodecanethiol-coated Au nanoparticles of two different sizes (average sizes are 5 nm and 2 nm) using nanogap electrodes, which have a well-defined gap size, at various temperatures. The Coulomb staircases and the Coulomb gap near-zero bias voltage caused by the suppression of the tunneling electrons due to the Coulomb blockade effect were observed in the current-voltage (I-V) curves of both sizes of nanoparticles at a low temperature (10 K). At room temperature, the Coulomb gap was observed only in the I-V curve of the smaller nanoparticles. This result indicates that the charging energy of the smaller nanoparticles is enough to overcome the thermal energy at room temperature. This suggests that it is possible to operate the SET devices at room temperature using the smaller nanoparticles as a Coulomb island.     

绿色长余辉材料MgAl_2O_4∶Mn~(2+)的合成及其发光特性

采用高温固相法在1350℃下合成了Mn2+掺杂的MgAl2O4发光材料,利用X射线衍射对所合成样品的结构进行了表征。用209nm的紫外灯照射样品后,观察到来自Mn2+的4T1-6A1跃迁的绿色长余辉发光。发光的激发光谱表明:Mn2+-3d组态内存在一系列强的激发峰,分别在279,361,386,427,451nm,同时还有209nm处的Mn-O电荷迁移带,激发该吸收带会产生很强的绿色余辉。测量了余辉的衰减曲线及热释光谱,分析了Mn2+掺杂浓度对样品余辉性质的影响,给出了余辉产生的可能模型。     

The effect of PS porosity on the structure,optical and electrical properties of ZnS/PS

ZnS films were deposited on porous silicon (PS) substrates with different porosities by pulsed laser deposition (PLD). The crystalline structure, surface morphology of ZnS films on PS substrates and optical, electrical properties of ZnS/PS composites were studied. The results show that, ZnS films deposited on PS substrates were grown in preferred orientation along β-ZnS (111) direction corresponding to crystalline structure of cubic phase. With the increase of PS porosity, the XRD diffraction peak intensity of ZnS films decreases. Some voids and cracks appear in the films. Compared with as-prepared PS, the PL peak of PS for ZnS/PS has a blueshift. The larger the porosity of PS, the greater the blueshift is. A new green light emission located around 550 nm is observed with increasing PS porosity, which is ascribed to defect-center luminescence of ZnS. The blue, green emission of ZnS combined with the red emission of PS, a broad photoluminescence band (450–750 nm) is formed. ZnS/PS composites exhibited intense white light emission. The I–V characteristics of ZnS/PS heterojunctions showed rectifying behavior. Under forward bias conditions, the current density is large. Under reverse bias conditions, the current density nearly to be zero. The forward current increases with increasing PS porosity. This work lay a foundation for the realization of electroluminescence of ZnS/PS and solid white light emission devices.     

On the excitation mechanisms of the infrared-excited visible luminescence in Yb,Er-doped fluorides

Lattices doped with Yb³⁺ and Er³⁺ can be excited by infrared radiation (970 nm) to give green and red emission. Information upon the excitation mechanisms of the infrared-excited visible luminescence is obtained from the emission spectra under 365 nm and under 970 nm excitation and from the excitation spectra of the green and red emission in the excitation region between 350 nm and 500 nm. This is demonstrated for a number of Yb³⁺, Er³⁺-doped fluorides showing a high infrared-to-visible conversion efficiency.