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Formation of Oxides and Their Role in the Growth of Ag Nanoplates on GaAs Substrates Yugang Sun,*,† Changhui Lei,‡ David Gosztola,† and Rick Haasch‡ Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439, and Center for Microanalysis of Materials, Frederick Seitz Materials Research Laboratory, UniVersity of Illinois, 104 South Goodwin AVenue, Urbana, Illinois 61801 ReceiVed June 2, 2008. ReVised Manuscript ReceiVed August 4, 2008 Simple galvanic reactions between highly doped n-type GaAs wafers and a pure aqueous solution of AgNO3 at room temperature provide an easy and efficient protocol to directly deposit uniform Ag nanoplates with tunable dimensions on the GaAs substrates. The anisotropic growth of the Ag nanoplates in the absence of surfactant molecules might be partially ascribed to the codeposition of oxides of gallium and arsenic, which are revealed by extensive data from electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy, during the growth of the Ag nanoplates. The electron microscopic characterization shows that each Ag nanoplate has a “necked” geometry, that is, it pins on the GaAs lattices through only a tiny neck (with sizes of 0.3 M), the surfaces of the nanoplates become flat and their thicknesses increase with an increase in the concentration. In addition, the sizes of Ag nanoplates increase with growth time, while their thicknesses increase in a stepwise fashion through polymerization and fusion of adjacent nanoplates.47 For example, Figure 1A and B shows the SEM images of samples obtained through the reaction between n-GaAs wafers and a 2 M AgNO3 solution for 0.5 and 2 min, respectively. The as-grown nanoplates protrude out of the surfaces of the GaAs substrates and exhibit smooth surfaces. Elemental analysis of individual nanoplates by employing electron dispersive X-ray spectroscopy (EDS) indicates that the nanoplates are composed of pure Ag without any contamination from the GaAs substrates.46 The sizes, which are determined by measuring the lengths of the orthographic projections of individual nanoplates along their longitudinal axes in the surfaces of the substrates, of the Ag nanoplates increase from 112 ((21) to 340 ((85) nm when the growth time increases
Sun et al.
from 0.5 to 2 min. On the other hand, their relatively uniform thicknesses keep essentially constant at an average value of ∼22 nm. The dimensional evolution of the Ag nanoplates upon growth time indicates that growing a Ag naonplate may start with the formation of a very small Ag crystal (i.e., nucleus) at a defect site of the GaAs surface because the defect site has higher surface energy, and thus higher reactivity, than adjacent flat areas. Defects usually exist on the surfaces of GaAs wafers after their native oxide layers are removed with acids (e.g., HF) in ambient environment.50 The existence of defects usually results from the surface roughness (Figure 2) of GaAs wafers. The lateral dimension of the small crystal might be around 22 nm, which is consistent with the thickness of the as-grown Ag nanoplate. Electron diffraction and synchrotron X-ray diffraction (Figure S1, Supporting Information) reveal that the Ag nanoplates have basal surfaces terminated by (111) facets and multiple twin planes parallel to the basal surfaces regardless of their sizes. The uniformity of the twinned crystalline structure of the Ag nanoplates during their growth process implies that the small Ag nuclei formed at the early reaction stage also include multiple (111) twin planes, which may (at least partially) provide the confinement to guide the anisotropic growth of small nuclei into two-dimensional plates with large sizes. Once the nuclei are formed through fast reduction of Ag+ with surface electrons of the n-type GaAs wafers,45 the sequent growth process is dominated by a so-called hole injection process because the reduction potential (EAg+/Ag ≈ 0.8 V versus a normal hydrogen electrode (NHE)) of the Ag+/Ag pair is higher than the valence band (i.e., surface potential of ∼0.32 V versus NHE) of the n-GaAs wafer under the reaction conditions reported in this work.51 Figure 1C gives the diagram of energy levels of the reaction system, in which Ag nuclei have been formed on the surface of the n-GaAs substrate. In the growth, each Ag+ injects a hole (h+) into the GaAs lattice through a nucleus. The Ag+ becomes a Ag atom to deposit on the lateral surfaces of the nucleus (Figure 1D). Because the upward bending of the valence band of the GaAs around the GaAs/AgNO3 solution interface, the injected holes will stay in the space charge region (SCR) (Figure 1C). The holes diffuse to the surface area uncovered with the Ag nuclei to oxidize the surface GaAs lattices with the assistance of water (from the aqueous AgNO3 solution). The continuous hole injection process deposits more and more Ag atoms onto the lateral surfaces of the nucleus because these surfaces have higher surface energies than the basal (111) surfaces, which always represent the crystalline planes with the lowest surface energy for face-centered cubic (fcc) metals.52 The internal multiple (111) twin planes in the nucleus might provide crystalline confinement to direct the growth of the nucleus into a nanoplate. At the same time, a layer of oxides of GaAs is generated on the GaAs surface, which prevents the formation of new nuclei and facilitates the growth of the existing nuclei into larger plates. The oxide layer also serves as a spacer to avoid direct contact between the Ag nanoplates and the GaAs lattices of the substrate except at the defect sites where the nuclei are formed. The interfaces between the Ag nanoplates and the GaAs substrates are important to reveal the growth process of the Ag nanoplates. In our experiments, a number of cross-sectional samples have been prepared through low-temperature ion milling (cooled in liquid nitrogen) and all the particles of each sample have been carefully examined with low- and high-magnification TEM. As shown in Figure 1A and B, the Ag nanoplates protrude (50) Loo, Y.-L.; Hsu, J. W. P.; Willett, R. L.; Baldwin, K. W.; West, K. W.; Rogers, J. A. J. Vac. Sci. Technol., B 2002, 20, 2853. (51) Mandler, D.; Bard, A. J. Langmuir 1990, 6, 1489–1494. (52) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153–1175.
Growth of Ag Nanoplates
Figure 2. (A) Low- and (B) high-magnification TEM images of a crosssectional sample, clearly showing that a Ag nanoplate pins on the GaAs lattices through a small post (inside the ellipse), that is, a defect site of the GaAs substrate. The other areas underneath the Ag nanoplate are covered with a continuous oxide film. The inset in (A) highlights how the nanoplate was cut by ion milling. Frame (B) highlights the area inside the white box in (A).
out of the surfaces of the GaAs substrates with random orientations, which are consistent with the continuity of the (111) reflection ring with uniform intensity in the X-ray diffraction pattern (Figure S1, Supporting Information). Regardless of orientation, each nanoplate has one edge contacting the substrate surface, and this edge is defined as the “contact edge”. When an as-synthesized sample is ion milled along the [110] crystalline direction of GaAs to form a thin slice for TEM observations, only the Ag nanoplates with contact edges parallel to the specific {110} crystalline plane of GaAs completely display their contact edges in TEM samples. Low magnification TEM images (Figure
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S2, Supporting Information) of cross-sectional samples indicate that most Ag nanoplates are trimmed and only a partial contact edge of each nanoplate is left in the TEM samples. By considering the orientations of nanoplates, the nanoplates with contact edges fully observed in the TEM (Figure 2 and Figures S3-S5, Supporting Information) can be determined. The low magnification images (Figure 2A and Figure S3, Supporting Information) clearly show that an essentially continuous amorphous layer (with a thickness of ∼2 nm) covers the whole substrate surface (regardless of the coverage with Ag) except the specific contact points with the Ag nanoplates. Although the exact composition of this thin layer is difficult to identify from the TEM characterization, XPS and Raman spectroscopy provide direct evidence that the film is composed of oxides of GaAs (sections 3.1 and 3.2). High magnification images (Figure 2B and Figures S4 and S5B-D, Supporting Information) reveal that the oxide layer stops only at the contact point (highlighted in Figure 2B and Figure S4B, Supporting Information) at which the Ag nanoplate attaches to the GaAs lattice through a tiny protruded post (i.e., defect site, highlighted by the ellipses) with a size of
