Organic nanoparticles display size-dependent absorption and fluorescence bands and single photon emission. The detailed understanding of these effects is hindered by the difficulty in the synthesis of organic nanocrystals, i.e. organic nanoparticles with an ordered molecular arrangement. A possibility that remains mostly unexplored is the synthesis of such nanocrystals on solid surfaces. In the same way in which crystalline inorganic nanodots can be epitaxially grown on suitable substrates under conditions in which 3D Volmer-Weber growth takes place, an organic system could in principle be devised such that the growth of crystalline 3D islands sets in before the completion of the first monolayer. In practice, however, for organic adsorbates deposited on inorganic substrates intermolecular interactions are much weaker than molecule-substrate interactions, thus promoting a layer-by-layer growth mode, and preventing the fabrication of isolated 3D nanocrystal.

Here we show that, upon deposition of cone-shaped subphthalocyanine (SubPc) molecules on Cu(111), isolated triangular nanocrystallites up to 3 ML high appear on the surface before the completion of the first monolayer. The different molecular layers show an alternating or antiferroelectric (AF) stacking of the molecular dipole moments. The structure of such nanocrystals can be explained by the joint effect of electrostatic (dipole-dipole) and dispersive (π-π) interactions. Although 1 ML-thick islands can also be found on the surface, the molecular arrangement in these areas is different from the geometry of the 1^st-layer molecules in the crystallites. We suggest that the formation mechanism of the organic nanocrystals is related to the existence of two different adsorption geometries, cone-up and cone-down, each of which sits on different molecular layers placed at different distances from the surface upon crystallite formation.

A significant challenge in fabricating organic thin film transistors is that of controlling and understanding the properties of the interface between the organic semiconducting layer and the dielectric. It has been observed that charge transport can be affected significantly by chemically altering the dielectric surface with self-assembled monolayers (SAMs). However, the effects of the molecular scale interactions between the organic molecule and the substrate remain unclear. Diindenoperylene (DIP) is a promising candidate for applications in organic thin film electronics owing to the ability to form highly ordered films with excellent electrical properties. Here, using supersonic molecular beam techniques and in situ real time synchrotron X-ray scattering, we have examined the adsorption dynamics of DIP on silicon dioxide (SiO₂) and SiO₂ modified with a number of SAMs. These SAMs included octadecyltrichlorosilane (ODTS), octyltrichlorosilane (OTS), fluoroctyltrichlorosilane (FOTS), and hexamethyldisilazane (HMDS), representing a range of molecular sizes and chemical terminations (-CH₃ vs. –CF₃). In this work, we make use of x-ray intensity oscillations at the so-called anti-Bragg position to extract the occupation (coverage) of the individual layers as a function of time. These coverage-exposure relationships give us a direct measure of the (relative) probabilities of adsorption as a function of coverage. For the conditions examined (E_i = 5.1-12.3 eV, T_s = 40 °C) on all the starting surfaces (SiO₂ and SAM/SiO₂) we observe a decrease in the probability of adsorption with increasing incident energy indicative of trapping-mediated adsorption. The probability of adsorption on these starting surfaces is dependent on both the chain length and the chemical composition of the SAM, where the probability of adsorption is greatest on the thickest organic layer, ODTS, followed by OTS, SiO₂, FOTS, and HMDS. Once all surfaces are covered by DIP, however, the effects of incident kinetic energy are greatly reduced, and trapping is very efficient over the range of kinetic energies examined. For organic layers of comparable chemical composition and density, the initial probability of adsorption depends strongly on the layer thickness, where trapping on ODTS is most efficient, and on HMDS, least efficient. In a selected set of cases we will compare our experimental results with recent results from molecular dynamics simulations to obtain insight into the possible molecular-scale mechanisms/events that may occur in these systems. One such event appears to be the undeniable role of direct molecular insertion into the thicker organic layers such as ODTS.

The growth mode, morphology, crystallinity and electronic structure of thin rubrene (5,6,11,12–tetraphenylnaphthacene: C₄₂H₂₈) films grown in ultra-high vacuum (UHV) conditions have been studied in situ in the low-energy electron microscope (LEEM).

Rubrene is attracting a considerable attention since it has shown promise in OTFT’s, with field-effect mobilities of the single-crystal FET devices surpassing that of amorphous silicon and even pentacene. However, still little is known how the electronic and chemical properties of the substrate affect the crystallinity of rubrene thin films.

When rubrene is deposited on clean semiconductor surfaces, such as Si(111)-7x7, its reacts with the dangling bonds, partially dissociating and forming a disordered wetting layer. The nucleation of the crystalline islands follows, but high nucleation density and slow surface diffusion, due to a rough and disordered interface, result in a poor crystallinity of the films.

Soft- and reactive landing (SL/RL) of mass-selected ions enables highly selective preparation of uniform thin films of a variety of complex molecules on surfaces. We have recently demonstrated covalent immobilization of several model peptides on the self-assembled monolayer (SAM) of N-hydroxysuccinimidyl ester terminated alkylthiol on gold (NHS-SAM) using RL. Another remarkable finding is that SL and RL can be utilized for purification of peptide conformations that may not be stable in solution on SAM surfaces. For example, while Ac-Ala₁₅-Lys peptide mainly exist in its β-sheet conformation in solution, SL and RL of this peptide on the surface results in immobilization of a very stable α-helical conformation. This study presents a pathway for preparation of conformationally-selected peptide arrays that cannot be prepared using traditional solution-phase approaches. Here we present SL of singly and doubly protonated peptides (Gramicidin S, Substance P, Ac-Ala₁₅-LysH⁺ and Ac-LysH⁺-Ala₁₅) onto inert SAMs of alkylthiol (HSAM) and fluorinated alkylthiol (FSAM) on gold, and RL of singly protonated diamines and multiply protonated dendrimers onto reactive NHS-SAM and COF-terminated SAM surfaces. The modified SAM surfaces were characterized using in situ real time infrared reflection absorption spectroscopy (IRRAS) and secondary ion mass spectrometry (SIMS). In situ analysis of substrates during and after the SL/RL processes is essential for understanding charge transfer phenomena, reactivity of soft-landed molecules, and modification of the secondary structure of the molecule as a result of interaction with the surface.

Intro

The soft-landing of ion cluster onto a surface is a novel approach for the deposition of specifically selected cluster. Our approach differs from current deposition/ soft landing techniques in that it allows specific molecule to be landed with a KE of around 1-5 eV where current soft-landing techniques are in the range around 30 - 50 eV. Large KE may not be ideal for the landing of metal clusters as the energy is translated into motion along the surface resulting in aggregation along defects in the surface. We have recently developed soft-landing system using a drift tube, high pressure (1 – 100 torr), and a modified substrate to create a soft-landing around 1 - 5 eV. Surface modifications can isolate clusters and protect them from aggregation. This soft-landing method allows for the characterization of a specific soft landed metal cluster on the surface via AFM, Raman Spectroscopy, and Electrochemistry.

Method

Our soft-landing instrument allows for the direct ablation of metal samples via laser or atmospheric plasma. Laser ablation of a metal sample by a pulsed Nd:YAG laser for duration of 1-3 hrs acts as our source for ion clusters. The instrument is operated at a pressure between 1-13 torr, with a voltage between 10-500 ±V that is applied uniformly across the drift tube. Helium is used as a buffer gas and acts as a means to control cluster formation. The high pressure narrows the clusters KE from 40 eV to about 1 eV through collisions via the buffer gas. Clusters travelling within the drift tube are separated based on collisonal cross-section. A pulsed split ring assembly at the end of the drift tube isolates specifically selected clusters directly on an unprepared or modified surface. The sample surface is then removed for characterization.

Data

Drift Tube spectra collected at 8 torr shows a cluster range of n = 1-8 along with corresponding Cu_nO. Clusters that have been landed on the surface are characterized using AFM. Images show an aggregation of the clusters in a striation patterns along the surface. The bridging across a defect instead of deposition within has also been seen. This clustering, aggregation, and growth shows a soft-landing has been made on the surface keeping the Cu cations and clusters intact. Self assembly of specifically isolated cluster groups has also been shown to occur on the surface of mica. Other characterization methods and various clusters may be specifically selected and landed with time allowing further development.

Instrument operates at pressure/temperature closer to ambient conditions than currently available, allowing ions to be soft landed on various modified surfaces for isolation and characterization.

A combination of ion beam and electron spectroscopy techniques have been used to precisely understand the initial oxidation of a multicomponent alloy system, Haynes®214®. Surface sensitive nature of these techniques has been rightly exploited to precisely identify different phases formed at different times of oxidation. Haynes^® 214^® is a high temperature, Ni-Cr-Al-Fe based alloy, widely used in a plethora of applications ranging from “clean firing” of ceramics to the gas turbine industry. Various reports exist in the literature, which describe the long term stability of this alloy under oxidizing environments at temperatures above 900^oC. Despite the number of applications and significant interest in this alloy, a clear mechanism of the early stage oxidation has not been documented. An effort is underway to estimate the initial nucleation and growth of oxide scale on this alloy in various environments (such as dry and humid air). A combination of ion and electron based techniques such as Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), particle induced x-ray emission (PIXE), secondary ion mass spectrometry (SIMS) and Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) were used to measure the initial nucleation of the oxide scale on this alloy. NRA was used to track the oxygen depth profile while PIXE and RBS were used to track the changes in matrix components of the alloy for the formation of oxide scale for as-low-as 1 min of oxidation. Chemical analysis of this early stage of oxidation formation was studied by AES and XPS. Chemical surface mapping obtained from Auger analysis indicated that the initial oxide formed is chromia which is completely dominated by alumina after 7 min of oxidation treatment at 900^oC in air. Various merits of this work will be discussed along with possible future scope for using Ion beam techniques for processes such as oxidation of metals, a most widely studied problem.