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The growth of Pt nanofilms on well-defined Au(111) electrode surfaces, using electrochemical atomic layer epitaxy (EC-ALE), is described here. EC-ALE is a deposition method based on surface-limited reactions. This report describes the first use of surface-limited redox replacement reactions (SLR(3)) in an EC-ALE cycle to form atomically ordered metal nanofilms. The SLR(3) consisted of the underpotential deposition (UPD) of a copper atomic layer, subsequently replaced by Pt at open circuit, in a Pt cation solution. This SLR(3) was then used a cycle, repeated to grow thicker Pt films. Deposits were studied using a combination of electrochemistry (EC), in-situ scanning tunneling microscopy (STM) using an electrochemical flow cell, and ultrahigh vacuum (UHV) surface studies combined with electrochemistry (UHV-EC). A single redox replacement of upd Cu from a PtCl(4)(2-) solution yielded an incomplete monolayer, though no preferential deposition was observed at step edges. Use of an iodine adlayer, as a surfactant, facilitated the growth of uniformed films. In-situ STM images revealed ordered Au(111)-(square root 3 x square root 3)R30 degrees-iodine structure, with areas partially distorted by Pt nanoislands. After the second application, an ordered Moiré pattern was observed with a spacing consistent with the lattice mismatch between a Pt monolayer and the Au(111) substrate. After application of three or more cycles, a new adlattice, a (3 x 3)-iodine structure, was observed, previously observed for I atoms adsorbed on Pt(111). In addition, five atom adsorbed Pt-I complexes randomly decorated the surface and showed some mobility. These pinwheels, planar PtI(4) complexes, and the ordered (3 x 3)-iodine layer all appeared stable during rinsing with blank solution, free of I(-) and the Pt complex (PtCl(4)(2-)).

Nanoprobes for MRI and optical imaging are demonstrated. Gd@C‐dots possess strong fluorescence and can effectively enhance signals on T1‐weighted MR images. The nanoprobes have low toxicity, and, despite a relatively large size, can be efficiently excreted by renal clearance from the host after systemic injection.

Studies of the electrodeposition of Se atomic layers on Au(111) and Au(110) are presented. Three electrochemical methods of forming Se atomic layers were investigated:  reductive deposition, oxidative stripping of bulk Se, and reductive stripping of bulk Se. The resulting Se atomic layers were studied using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). LEED indicated the formation of Au(111)(√3×√3)R30°-Se and Au(110)(2×3)-Se structures. STM analysis confirmed the presence of those structures along with several others. At low Se coverages on Au(111), a mosaic structure was formed, composed of a large number of small domains of a (√3×√3)R30°-Se structure, separated by areas void of Se. At higher coverages, near 1/3, the (√3×√3)R30° structure covered most of the surface, except for a number of linear phase boundaries. Commensurate with completion of the (√3×√3)R30° structure, some domains of square Se8 rings were usually evident, as well. At still higher coverages, a heterogeneous surface was formed, composed of a complex network of rings, chains, clusters, and pits. This heterogeneity appears to result from slow deposition kinetics, probably the result of both a low exchange current and low Se surface mobility. Some of the kinetic sluggishness may have resulted from the need to translate whole domains of Se atoms from one site to another, in order to remove phase boundaries. STM studies of the Au(110) surface indicated that only the (2×3) structure was formed at coverages much below 1 monolayer and that it was formed homogeneously. At monolayer coverages and above, a honeycomb structure composed of chains of Se atoms was observed, which filled in at still higher coverages to complete a second Se layer.

Thin films of CdTe, CdSe, and CdS have been electrodeposited by electrochemical atomic layer epitaxy (ECALE), using an automated electrochemical deposition system. Previous reports of an automated system for forming ECALE deposits involved use of a small thin layer flow cell, which revealed several drawbacks. Conversion of the thin layer cell to a thick layer design resulted in greatly improved deposit quality and reproducibility. Deposits were analyzed using electron probe microanalysis (EPMA), scanning electron microscopy (SEM), and grazing incident X-ray diffraction (XRD). The results were consistent with a layer by layer growth mode and the principles of atomic layer epitaxy. CdTe films were grown using up to 1000 ECALE cycles, and were stoichiometric through 500. The 1000 cycle films were a few percent rich in Te, under the conditions used. CdSe and CdS films formed also contained some excess chalcogenide, probably the result of less then ideal deposition parameters. Increasing amounts of particulates and surface roughening were observed for the 500 and 1000 cycle CdTe and CdSe films, relative to the 200 cycle deposits normally formed. This roughening may result from the excess chalcogenide. XRD of the films indicated cubic crystal structures with preferred (111) orientations for all three compounds.

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The electrodeposition of germanium on Au(111) in aqueous solutions has been investigated by means of cyclic voltammetry, Auger electron spectroscopy, and in situ scanning tunneling microscopy (STM). The data yield a picture of germanium deposition, which starts with the formation of two well-ordered hydroxide phases, with 1/3 ML and 4/9 ML coverages upon initial reduction of the Ge(IV) species (probably H(2)GeO(3) at pH 4.7). Those structures appear to result from a three-electron reduction to form surface-limited structures with (square root(3) x square root(3))R30 degrees or (3 x 3) unit cells, respectively. Further reduction, probably in a two-electron process from the hydroxide structures, resulted in a germanium hydride structure, again surface-limited, with a coverage of close to 0.8 ML. The hydride structure is very flat, though with the periodic modulation characteristic of a Moiré pattern. Longer deposition times and lower potentials resulted in increased coverage of Ge in some cases, but with apparently limited coverage as a function of pH. The maximum Ge coverage, about 4 ML, was observed using a pH 9.32 deposition solution. At potentials negative of the Moiré pattern, about -850 mV versus Ag/AgCl, a "corruption" of the smooth Moiré pattern occurred. This roughening appears to mark the initial formation of a Au-Ge alloy, accounting for the observation of coverage in excess of that needed to form the Moiré pattern at some pH values.

The method of electrochemical atomic layer epitaxy (ECALE) is described. It involves the alternated electrochemical deposition of atomic layers of elements to form compound semiconductors. It is being investigated as a method for forming epitaxial thin films. Presently, it appears that the method is applicable to a wide range of compound semiconductors composed of a metal and one of the following main group elements: S, Se, Te, As, Sb, or Bi. Initial studies have involved CdTe deposition. Factors controlling deposit structure and composition are discussed here. Preliminary results which show that ordered electrodeposits of CdTe can be formed by the ECALE method are also presented. Results reported here were obtained with both a polycrystalline Au thin-layer electrochemical cell and a single-crystal Au electrode with faces oriented to the (111), (110), and (100) planes. The single-crystal electrode was contained in a UHV surface analysis instrument with an integral electrochemical cell. Deposits were examined without their exposure to air using LEED and Auger electron spectroscopy. Coverages were determined using coulometry in the thin-layer electrochemical cell.