Metallic nanometer clusters composed of 55 gold atoms isolated from each other by a thin organic ligand film, with a total diameter of 2.1 nm, were fabricated as monolayers on various technically relevant substrates by using a two-step self-assembly (SA) procedure as well as combined Langmuir-Blodgett (LB) and SA techniques. Scanning tunneling microscopy (STM) and scanning force microscopy (SFM) studies on these systems reveal a short-range close packing of these clusters. Single-electron tunneling (SET) phenomena, including Coulomb blockade and Coulomb staircase, are studied by local tunneling spectroscopy (LTS) at room temperature as well as at low temperature (90-100 K). A narrow spread of the charge energy is observed on such prepared cluster monolayers.In recent years, nanometer-sized clusters of metals separated from each other by organic ligand shells have received increasing scientific interest [1]. These clusters have a welldefined size in the metal core, with a fixed number of metal atoms, and are stabilized by the organic ligand shell. Thus, they provide excellent model systems for monodispersed metal clusters, embedded in a dielectric matrix for the investigation of physical properties related to nanoscale particles. Among the various clusters, Au 55 (PPh 3 ) 12 Cl 6 (in short Au 55 ) and Pt 309 Phen 36 O 30 (in short Pt 309 ), have been intensely studied by different groups [2-6]. The possible electronic application of these metal clusters, with diameters of 1.4-2.1 nm, as "quantum dots" was first discussed by Schön and Simon [7].Scanning tunneling microscopy and local tunneling spectroscopy has been applied to measure the electronic properties of compact Au 55 cluster pellets [8] or solvent-evaporated Au 55 and Pt 309 clusters on various substrates [9, 10]. Singleelectron tunneling on these systems has been clearly observed in the current-voltage (I-V ) curves. In previous investigations, the cluster solutions were dropped on a substrate and then dried, or the clusters were compressed to a pellet. The * Corresponding author quantitative analysis of I-V curves is based to some extend on the undefined geometric packing of the clusters. The charge energy had a wider spread than expected [10]. Further, in LTS investigations, one interesting question is addressed: whether the additional structures on the Coulomb staircase measured at 4.2 K might be induced by the interaction between the substrate and the clusters or by that between the ligand molecules and the metal core [10]. A uniform packing of clusters on the substrate will help to rule out some unknown effects for the quantitative analysis, since modification of the charging energy of a cluster might be affected by its environment. In addition, achieving ordered arrays of these metal dots is an essential step for developing nanoscale electronic devices [11].Recently, the closed packing of gold clusters, each encapsulated by a monolayer of alkyl thiol molecules, has been successfully achieved by spin casting [12]. As is known, spin casting of suspens...
15 % denaturing polyacrylamide gel. The ApA hydrolysis by the 2:1 Zn II ± TPBA complex was followed by reverse-phase HPLC. The 2',3'-cyclic monophosphate of adenosine as the intermediate was rapidly hydrolyzed to the 2'-and 3'-monophosphates; thus, not much intermediate accumulated.
Using a scanning tunneling microscope operated in a spectroscopic mode we imaged flux-line lattices in niobium diselenide at various external magnetic fields. From the evaluation of a large number of tnnnelingcurrent profiles taken across the individual vortices we deduced the dependence of the vortex-core radius on the applied magnetic field. It was found that the core radius shows a pronounced decrease with increasing field, even for H/Hc2 << 1. This behavior is qualitatively well characterized by self-consistent solutions of the Usadel equations.The outstanding analytical potential of scanning tunneling microscopy (STM) is due to its inherent capability of obtaining spectroscopic information at the sub-meV level combined with a lateral resolution of the order of angstroms. This enables one to probe superconductive properties on a length scale smaller than the coherence length. The present investigations have been performed on 2H-NbSe 2 which is a layered type-II superconductor. It is particularly well suited for STM because the freshly cleaved surfaces are relatively inert against contamination.The experiments have been performed with a low-temperature STM which has been described in some detail in Ref. 1. The local surface density of quasiparticle states is probed in the following way [2]: At any point on the surface the STM's feedback loop is interrupted for a short period (< 1 second), while the tunneling voltage V is decreased from beyond the superconductor's gap voltage Vg = Ao/e to a certain value V o < Vg. Upon raster-scanning the normal conducting tip the corresponding tunneling current I(Vo) then continuously varies between an upper limit Ima x right at the center of a vortex and a lower limit Imi n farthest from all adjacent vortices. All present measurements have been performed at Tfr c = 0.6. To change the vortex spacing the external magnetic field perpendicular to the sample surface was varied within the interval 0 _< WHc2_< 0.3. Figure 1 shows a tunneling-current image of the vortex lattice at an external field of H/Hc2 = 0.14. The sub-gap tunneling voltage was Vo/Vg = 0.88 at negative sample bias.We systematically analyzed numerous line scans across individual vortices. The actual vortex profiles were deduced from complete images such as displayed in Fig. 1. A characteristic measure of the respective vortex-core radius is arbitrarily defined by the particular distance p from the vortex center
Kleine Silbercluster (primär vermutlich Ag4‐Cluster, die zu Agn (400
The vortex lattice in clean 2H-NbSe2 is investigated by STM at 4.2 K. The experimental results predominantly address three major issues of current importance. (i) Sufficiently large in-plane transport currents through the superconducting sample cause the motion of vortices, which can directly be detected. (ii) The apparent vortex diameter is about twice that expected from elementary theory. A pronounced shrinking of the vortex core with increasing external magnetic field is observed. (iii) The STM results combined with some elementary magnetostatic model calculations shed some light on the difficulties encountered in the imaging offiux line lattices by magnetic force microscopy. . INTRODUCTIONThe outstanding analytical potential of STM is due to the capability of obtaining spectroscopic information at sub-meV resolution with a spatial resolution of the order of angstroms. One is thus able to measure superconducting properties on a length scale smaller than the coherence length.Nevertheless, it appears that progress in this field is relaltively slow1 mainly because of two reasons: (i) The cryogenic environment involves considerable technical constraints, e.g., with respect to the vibration isolation of the STM, in situ surface preparation and characterization, and the achievement of satisfactory UHV conditions. (ii) Almost all interesting materials usually have ill-defined surfaces which can even be insulating or semiconducting due to oxigen diffusion.A notable exception is the layered type-il superconductor 2H-NbSe2, which can easily be cleaved and which is relatively inert against contaminations.2 The magnetic properties ofthis superconductor may be of some interest due to the pronounced anisotropy in its behavior as a type-TI compound.3 2H-NbSe2 exhibits an in-plane coherence length of (O) = 7.7 nm and a London penetration depth of (O) 69 nm.Hence, a magnetic field perpendicular to the atomic layers experiences a Ginzburg -Landau parameter of i = 9 which classifies 2H-NbSe2 as a fairly hard type-Il superconductor. Along the crystalline c-axis, the coherence length is smaller by a factor of about 3 .3, while the penetration depth is increased by about a factor of 3 .3 .4 Since the c-axis lattice spacing is 1 .3 nm the coherence length perpendicular to the hexagonal layers extends over several atomic Se and Nb layers. 2H-NbSe2 can thus be considered as a good three-dimensiOnal anisotropic superconductor, which should not involve any vortex anomaly as known from the high-Ta materials.5 The critical temperature is T = 7.2 K.6 Our STM experiments yielded a gap width of A (4.2K) = 1.1 meV obtained by averaging A as a BCS fit parameter over 100 1(V) curves.All experiments have been performed with a simple, reliable low-temperature STM which is described in some detail in Ref. 7. While measurements below 1 K are, due to the improved spectroscopic 140
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