SnS2 and SnSe2 thin films were deposited by molecular beam epitaxy (MBE) methods on a variety of layered semiconductor substrates (freshly cleaved SnS2, SnSe2, WSe2, MoS2, MoTe2, GaSe, InSe) and cleaved mica, for investigation of the interfaces formed as a result of MBE growth. These ultrathin films were characterized in situ by x-ray photoelectron spectroscopy and surface reflection high energy and low energy electron diffraction techniques. The growth modes were verified ex situ by scanning tunneling microscopy and/or atomic force microscopy [scanning probe microscopies (SPM)]. Despite the chemical and structural similarities between SnS2 and SnSe2, SPM measurements show that the two materials as ultrathin films have different growth morphologies in the first few monolayers. Photoelectron spectroscopy (PES) measurements lead to the conclusion that both materials grow on the basal planes of most layered substrates in an epitaxial, layer-by-layer mode. Small deviations from ideal Frank–van der Merwe growth for these thin films could be determined from PES intensity ratios versus coverage. Deposition of both materials on freshly cleaved mica, where the lattice mismatch between the substrate and the overlayer surface unit cell dimension exceeds 40%, yields exclusively 500–1000 Å diameter epitaxial islands with threefold symmetry.

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The epitaxial growth of fluoroaluminum-phthalocyanine (FAlPc) thin films on the hep (0001) tin disulfide (SnS2) surface is reported. Growth conditions can be achieved that lead to FAlPc films of polymer-like aggregates (FAlPc)" with the long axis of the aggregate apparently standing perpendicular to the surface. When the substrate is held at a temperature of 260 °C during deposition, reflection high-energy electron diffraction of the resultant thin films showed that FAlPc grows epitaxially on SnS2 in a square lattice structure, starting with the first monolayer, with a unit cell dimension of 13.2 ± 0.8 A. One axis of the resulting FAlPc square lattice overlayer is oriented along the main axis of the basal plane SnS2. Scanning tunneling microscopy (STM) showed elongated crystallites growing along the main crystallographic axes of the substrate, and these same STM data indicate that phthalocyanine film growth may be initiated at step sites on the SnS2 surface. The development of H-aggregate (FAlPc)" in these thin films is confirmed by the position of the Q-band absorbance maximum observed for these thin films (642 nm), which is blue-shifted from the monomer absorbance maximum at ca. 675 nm. The Q-band spectrum of the epitaxial film is much narrower (fwhm 28 nm) than comparable spectra from less-ordered thin films of (FAlPc)" aggregates deposited on sapphire (fwhm 55 nm). The polarization dependence of these Q-band spectra was consistent with individual FAlPc molecules in these thin films, on average, lying parallel to the surface plane. Dye sensitization/photoelectrochemistry showed photocurrent action spectra that paralleled the absorbance spectra and made possible the characterization of even monolayer-level coverages. Quantum efficiencies for charge injection from the FAlPc aggregates into the SnS2 conduction band (<5%) was lower by at least a factor of 10, relative to previously explored phthalocyanine sensitizers which do not form linear cofacial aggregates.

Lesson: Scanning Probe Microscopy: "feeling" what you can't see at the nanometer scale Standards:HS-PS1-3. Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.HS-PS2-6. Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.HS-PS2-1. Analyze data to support the claim that Newton's second law of motion describes the mathematical relationship among the net force on a macroscopic object, its mass, and its acceleration.HS-ETS1-2. Design a solution to a complex real-world problem by breaking it down into smaller, more manageable problems that can be solved through engineering.

Simple, low-maintenance instrumentation can be used to image virtually any material at the microscopic and nanoscopic level Scanning probe microscopies are quickly becoming routine methods in many laboratories. The ability to probe the microscopic and nanoscopic structure of surfaces in a variety of ambient conditions with a lowmaintenance instrument that sits on a benchtop has contributed to the popularity of these techniques. Scanning tunneling microscopy (STM), invented in 1982, was the first technique capable of directly imaging surface atoms in real space (1), Although this instrument has been miniaturized, multiplexed, and adapted to many environments, there have been few major innovations since its invention.The same cannot be said of atomic force microscopy (AFM), invented in 1986 (2). Whereas the scanning tunneling