Quantum-confined InP nanocrystals from 20 to 50 Å in diameter
have been synthesized via the reaction of
InCl3 and
P(Si(CH3)3)3 in
trioctylphosphine oxide (TOPO) at elevated temperatures. The
nanocrystals are
highly crystalline, monodisperse, and soluble in various organic
solvents. Improved size distributions have
been obtained by size-selectively reprecipitating the nanocrystals.
The UV/vis absorption spectra of the particles
show the characteristic blue shift of the band gap of up to 1 eV due to
quantum confinement, a moderately
well-resolved first excitonic excited state, and, in some cases, the
resolution of a higher excited state.
Structurally, the nanocrystals are characterized with powder X-ray
diffraction and transmission electron
microscopy. Raman spectroscopy reveals TO and LO modes near the
characteristic bulk InP positions as
well a surface mode resulting from finite size. The Raman line
widths, line positions, and relative intensities
are all size-dependent . X-ray photoelectron spectroscopy (XPS)
shows the nanocrystals have a nearly
stoichiometric ratio of indium to phosphorus with TOPO surface
coverages ranging from 30% to 100%. We
have also used XPS to correlate the oxidation of the nanocrystal
surface with photoluminescence intensity.
Photoluminescence is observed as both band edge and deep trap
emission with both features shifting with
nanocrystal size. The luminescence is highly dependent on the
surface of the nanocrystal with oxidation
being a necessary condition for emission.
We present electrical measurements of single Au and CdSe nanocrystals. The devices are fabricated using a hybrid scheme which combines electron beam lithography and wet chemistry to bind nanocrystals in tunneling contact between two closely spaced metallic leads. The current–voltage characteristics of these devices exhibit a Coulomb staircase with a charging energy of ∼50 meV. This technique is readily adapted to the study of a host of nanocrystals made by solution chemistry.
We discuss the use of a conducting-tip atomic force microscope (AFM) for the imaging and electrical measurement of chemically derived nanostructures. First, scanning probe microscopy of CdSe and Au nanocrystals bound to a substrate with a self assembled monolayer will be discussed. It is found that imaging in liquids is necessary to avoid removing the nanocrystals. We then address some issues in performing electrical measurements in liquids. In particular, we examine the conducting properties of the AFM tip when imaging a flat surface, highly oriented pyrolytic graphite, in a non-polar liquid, hexadecane. We find that the solvation layers between the tip and the substrate strongly influence the electrical properties.
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