Single Crystal Interface Research Articles

Spatial Anisotropy of Charge Transfer at Perfluoropentacene-Pentacene (001) Single-Crystal Interfaces and its Relevance for Thin Film Devices.

Archetypal donor-acceptor (D-A) interfaces composed of perfluoropentacene (PFP) and pentacene (PEN) are examined for charge transfer (CT) state formation and energetics as a function of their respective molecular configuration. To exclude morphological interference, our structural as well as highly sensitive differential reflectance spectroscopy studies were carried out on PFP thin films epitaxially grown on PEN(001) single-crystal facets. Whereas the experimental data supported by complementary theoretical calculations confirm the formation of a strong CT state in the case of a cofacial PFP-PEN stacking, CT formation is energetically less favorable and thus absent for the corresponding head-to-tail configuration as disclosed for the first time. In view of technological implementations, the knowledge gained on the single-crystal references is transferred to thin-film diodes composed of either stacked PFP/PEN bilayers or mixed PFP:PEN heterojunction interfaces. As demonstrated, their electronic and electroluminescent behavior can be consistently described by the absence or presence of interfacial CT states. Thus, our results hint at the thorough design of D-A interfaces to achieve the highest device performances.

Perfect Strain Relaxation in Metamorphic Epitaxial Aluminum on Silicon through Primary and Secondary Interface Misfit Dislocation Arrays.

Understanding the atomically precise arrangement of atoms at epitaxial interfaces is important for emerging technologies such as quantum materials that have function and performance dictated by bonds and defects that are energetically active on the micro-electronvolt scale. A combination of atomistic modeling and dislocation theory analysis describes both primary and secondary dislocation networks at the metamorphic Al/Si (111) interface, which is experimentally validated by atomic resolution scanning transmission electron microscopy. The electron microscopy images show primary misfit dislocations for the majority of the strain relief and evidence of a secondary structure allowing for complete relaxation of the Al-Si misfit strain. This study demonstrates the equilibrium interface that represents the lowest energy structure of a highly mismatched and semicoherent single-crystal interface with complete strain relief in an atomically abrupt structure.

Molecularly clean ionic liquid/rubrene single-crystal interfaces revealed by frequency modulation atomic force microscopy.

The structural properties of ionic liquid/rubrene single-crystal interfaces were investigated using frequency modulation atomic force microscopy. The spontaneous dissolution of rubrene molecules into the ionic liquid was triggered by surface defects such as rubrene oxide defects, and the dissolution rate strongly depended on the initial conditions of the rubrene surface. Dissolution of the second rubrene layer was slower due to the lower defect density, leading to the formation of a clean interface irrespective of the initial conditions. Molecular-resolution images were easily obtained at the interface, and their corrugation patterns changed with the applied force. Force curve measurements revealed that a few solvation layers of ionic liquid molecules formed at the interface, and the force needed to penetrate the solvation layers was an order of magnitude smaller than typical ionic liquid/inorganic solid interfaces. These specific properties are discussed with respect to electric double-layer transistors based on the ionic liquid/rubrene single-crystal interface.

Electrostatic Surface Charge at Aqueous/α-Al2O3Single-Crystal Interfaces as Probed by Optical Second-Harmonic Generation

Second harmonic generation (SHG) spectroscopy was used to characterize the pH-dependent electrostatic charging behavior of (0001) and (102) crystallographic surfaces of corundum (alpha-Al2O3) single-crystal substrates. The pH value of the point of zero charge (pH(pzc)) for each surface was determined by monitoring the SH response during three consecutive pH titrations conducted with 1, 10, and 100 mM NaNO3 carbonate-free aqueous solutions. The crossing point of the three titration curves, which corresponds to the pH(pzc), occurs at pH 4.1 +/- 0.4 for the (0001) surface and pH 5.2 +/- 0.4 for the (102) surface. SHG measurements that were recorded as a function of NaNO3 concentration at fixed pH values were found to corroborate the pH(pzc) values identified in the pH titrations. A comparison of the SHG results with surface protonation constants calculated using a simple electrostatic model suggests that surface relaxation and bonding changes resulting from surface hydration do not account for differences between experimental observations and model predictions. The measured pH(pzc) values for the alpha-Al2O3 single-crystal surfaces are significantly more acidic than published values for Al-(hydr)oxide particles which typically range from pH 8 to 10. This discrepancy suggests that the charging behavior of Al-(hydr)oxide particles is determined by surface sites associated with defects assuming that differences in surface acidity reflect differences in the coordination environment and local structure of the potential-determining surface groups.

Isotropic to Nematic Transition in Solutions of Cylindrical PB−PEO Block Copolymer Micelles Close to a Wall

The influence of a flat interface on the isotropic to nematic (I/N) phase transition was investigated for aqueous solutions of cylindrical micelles of polybutadiene−poly(ethylene oxide) block copolymers using specular neutron reflectivity. While the I/N transition in the bulk occurs at the solute volume fraction φN/I, we observed the formation of a condensed layer close to a silicon single-crystal interface at volume fractions of about φi ≈ 0.85φI/N. The thickness of this layer does not vary if the solute fraction is changed, while its density increases drastically within a very narrow range of volume fractions. This observation is interpreted as the formation of a nematically ordered layer which is induced by the interface.

Electrochemistry of the I-on-Pd single-crystal interface: studies by UHV-EC and in situ STM

A single chemisorbed layer of zerovalent I atoms has been found to enhance, at ambient temperatures, the reactivity of Pd electrode surfaces. Three unique reactions (anodic dissolution in non-corrosive electrolyte, regeneration of well-ordered single-crystal surfaces, and “electrochemical digital etching”) have been investigated at Pd(111) and Pd(100) single-crystal electrodes and are described in this paper. Experimental measurements were based upon a combination of electrochemistry (EC), low-energy electron diffraction (LEED), and in-situ scanning tunneling microscopy (STM).

Liquid and solid phase regrowth of Si by laser irradiation and thermally assisted flash annealing

Abstract Ion implanted and deposited Si layers have been recrystallized using pulsed Nd:YAG and cw Ar ion laser irradiation and thermally assisted flash annealing. By use of Rutherford backscattering and channeling. TEM. SEM and interference optical microscopy techniques, two distinct regrowth regimes have been established. I. Liquid phase where regrowth occurs from the underlying solid-liquid interface. In this regime the regrowth layer is free of extended defects, dopant profiles are consistent with liquid diffusion and equilibrium solid solubilities can be exceeded. II. Solid phase where regrowth occurs from the amorphous single-crystal interface. No dopant redistribution is observed.

Epitaxial regrowth of Ar-implanted amorphous silicon

The influence of Ar on the recrystallization of implanted amorphous silicon layers has been studied. Investigation of the growth kinetics at temperatures up to 575 °C showed that epitaxial regrowth of silicon is halted if the Ar concentration at the amorphous–single-crystal interface reaches about 1.5×1020 cm−3. Further regrowth could be obtained by higher-temperature annealings, during which the Ar concentration profile was changed and part of the Ar diffused out. TEM analysis showed, that in samples with a dose of 6×1015 Ar ions/cm2, the formation of Ar bubbles occurred during implantation. During annealing the Ar bubbles grew about a factor of 6 in diameter. The epitaxially regrown silicon layers contained a large amount of microtwins. We present a model which relates the occurrence of microtwins to the existence of Ar bubbles.