Session J17: Focus Session: Surfaces and Interfaces in Nonoxide Nanostructures: Growth, Structure, and Characterization - Organic/Inorganic Interfaces & Quantum Dots

Current at Metal-Organic Interfaces

Charge transport through atomic and molecular constrictions greatly affects the operation and performance of organic electronic devices. Much of our understanding of the charge injection and extraction processes in these systems relays on our knowledge of the electronic structure at the metal-organic interface. Despite significant experimental and theoretical advances in studying charge transport in nanoscale junctions, a microscopic understanding at the single atom/molecule level is missing. In the present talk I will present our recent results to probe directly the nanocontact between single molecules and a metal electrode using scanning probe microscopy and spectroscopy. The experiments provide unprecedented microscopic details of single molecule and atom junctions and open new avenues to study quantum critical and many body phenomena at the atomic scale. Implications for energy conversion devices and carbon based nanoelectronics will also be discussed.   

Temperature dependence in metal/organic heteroepitaxy

The nucleation and growth of 2D single layers of tetraphenyl porphyrin molecules on Ag(111) are studied with variable temperature scanning tunneling microscopy. The heteroepitaxy of the organic/metal thin film occurs in strict analogy with known processes of metal heteroepitaxy. A similar heirarchy of energetic barriers to diffusion along edges and around corners is established. Temperature is the key component to selectively activating these barriers and determining shape of the adislands, from fractal-like shapes at low temperature to compact shape at high temperatures. Using existing models of metal heteroepitaxy, the terrace diffusion and binding energies of tetraphenyl porphyrin are approximated from measurement of island size as a function of temperature. This study provides evidence of the validity of using existing models of metal heteroepitaxy for the description of organic/metal heteroepitaxial systems.   

Scanning Tunneling Microscopy investigation of the assembly of diF-TES-ADT on Ag(111)

Over the past two decades organic molecules have shown increasing promise as active layers in electrical devices such as field effect transistors, organic light emitting diodes, and organic photovoltaic devices. The suitability of organic molecules for use in these devices is governed by several properties, chief among them being the ability to self-organize into a film showing high carrier mobility. Organic thin film transistors (OTFT) partially composed of solution processed 2,8-difluoro-5,11-bis(triethylsilylethynyl)-anthradithiophene have shown high performance. To date these OTFTs have been constructed solely by solution processing. As such, we have chosen to investigate the possibility of vapor deposition as an alternative. Our investigation of the viability of vapor deposition of this promising molecule begins with deposition on Ag(111) as a model system. Preliminary STM results will be presented and discussed.   

DFT study of metal/organic interfaces; optimizing morphology and energy levels for maximum Voc

Metal/organic interfaces are important for understanding the electronic properties of any device incorporating organic molecules, although these interfaces have been much less studied than their metal/inorganic counterparts. Using density functional theory, we examine the electronic structure of the interfaces of three metals: silver, aluminum, magnesium, and an organic molecule, Alq3, which is utilized in organic light emitting diodes. We calculate properties of interfaces with a clean metal surface as well as ones with small metal particles injected close to the organic. The effects of the different metals are to charge the Alq3 to varying degrees and perturb the energy levels as the metal states mix with the organic molecule. Insights into the energy level alignment and morphology at these interfaces as a function of the electrode workfunction will be discussed, with the goal of maximizing the open circuit voltage through the choice of metal and deposition process.   

DFT based modeling of C60/Dichloropentacene/Au OPV heterojunctions

The co-assembly of functionalized pentacenes (electron-donor materials) and fullerenes (electron-acceptor materials) on metal substrates provides a model for studying the structural and electronic properties for novel organic photovoltaic (OPV) heterojunctions. Our previous STM experimental results show C$_{60}$ to form single, double and triple nano chains on an intact single-domain, brick-wall structured 6,13-dichloropentacene (DCP) monolayer adsorbed on stepped Au(788). Here, we present theoretical DFT calculations of the geometric and electronic structure, and the charge transfer in this interacting three-component system. Our calculations show that single C$_{60}$ molecules prefer to either absorb on top of the DCP molecules (slightly shifted off the Cl center) or in between the DCP rows of the brick-wall structure. When adsorbing chains of C$_{60}$ they will align with either the troughs in between the DCP rows or the top of the DCP rows, in agreement with experiment. Compared to the isolated DCP molecules, the HOMO and LUMO levels move up towards the vacuum level by about 1 eV upon monolayer formation, resulting in charge transfer to C$_{60}$.   

Acetophenone on Si(001) with STM and DFT

Organic molecules are likely to play an important role in future technologies, e.g., in novel devices where individual molecules are incorporated as active elements, and in extending the functionality of existing technologies. A detailed, atomic-scale understanding of the structural and electronic properties of molecules on surfaces is key to the development of these technologies. Here we present scanning tunnelling microscopy (STM) and density functional theory (DFT) data of the surface binding configurations of acetophenone adsorbed to Si(001). Topographic and spectroscopic tunnelling experiments were performed at 77~K and room temperature in the limit of very low coverage. We find in analogy to other similar molecules such as acetaldehyde~[1], acetone~[2,3] and acetic acid~[4], acetophenone molecules covalently bond to the Si(001) surface in a variety of configurations that can be directly manipulated using the STM tip. In its most stable configuration, the adsorbate stands upright on the surface, attached via the C and O atoms of its acetyl group, producing a geometry that is robust and attractive for molecular electronics applications. [1] JCP 131, 104707 (2010), [2] PCCP 11, 2747 (2009), [3] JACS 129, 11402 (2007), [4] PRB 84, 153302 (2011).   

Contact Angle Behavior for Fullerene/Porphyrin Mixtures on Si surfaces

Fullerene/porphyrin mixtures are of great interest in bulk heterojunction organic solar cells. Here we study the morphology of the phase separation which occurs when [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) and tetranitro-zinc phthalocyanine (tn-ZnPc), are deposited onto silicon (111) substrates, including the individual domain length scales, shapes and wetting angles. tn-ZnPC forms small clusters on the Si surface with a contact angle of approximately 15\r{ }, while PCBM forms compact clusters on broad ($\sim $0.5 um diameter) ``wetting-layer'' disks, with the cluster contact angle of $\sim $19\r{ }. Interestingly, a 50{\%} mixture shows topography qualitatively similar to that for PCBM, but with a larger contact angle of 22\r{ }, indicating that the mixture wets the interface less than either pure component alone.   

Molecular nanostructures on graphite

The self-assembly of different amino acids on graphite has been studied using Scanning Tunneling Microscopy. Experiments involving the amino acid methionine have shown that the molecules arrange themselves in well-ordered molecular wires with equidistant spacing tunable by the amount of adsorbate concentration on the surface. This behavior can be explained by an attractive interaction of the amino and carboxyl groups with each other, whereas the side chains exhibit repulsive interactions. Experiments using other amino acids with different side chains, like tyrosine and histidine, show adsorption behavior which lead to densely packed films of well-ordered amino acids, but no molecular wire structure. The repulsive interactions of the side chains can not be experimentally observed. This interesting phenomenon of inter-molecular interaction was further investigated using molecular mechanics calculations for these molecules.   

Spontaneous Formation of Quantum Height Manganese Gallium Islands and Atomic Chains on N-polar Gallium Nitride (000\underline {1})

Significant interest has been shown over the last 15+ years in the growth of 2-D island nanostructures of special heights on semiconductor surfaces due to quantum size effects, often referred to as `electronic growth.' Recently, there has been much interest in growth of magnetic metal layers on gallium nitride surfaces, but electronic growth in this system was not reported, until now. Surprisingly, we find that deposition of manganese onto gallium-rich, GaN(000\underline {1}) results in the spectacular formation of 2-D quantum-height MnGa island structures. Two unique island heights, differing by just one atomic layer, are observed - one being 0.93 nm (5 atomic layers), the other 1.13 nm (6 atomic layers). The 0.93 nm high islands are unstable against the completion of the next atomic layer (0.93 + 0.20 nm), so a single quantum thickness is preferred (1.13 nm). A row structure at the surface of the islands is also revealed, with atomic resolution images suggesting a mixture of Mn and Ga. Auger electron spectroscopy confirms a significant surface Mn content. In addition, growth of 1-D atomic chains at the surface of the completed 1.13 nm high islands is also seen, indicating strongly anisotropic diffusion. The observed behavior is consistent with a quantum size effect driven growth.   

Wavelength tunable high quality positioned InAs quantum dots grown on patterned GaAs (001) substrates

Semiconductor quantum dots serve as ideal contenders in the domains of quantum optics, quantum cryptography and quantum information processing. The decisive factor about their possible applications is their peak emission energy. Naturally grown quantum dots suffer from the problem of random nucleation behavior and non-uniform dot sizes. Here we report on the growth of site-controlled InAs quantum dots on pre-patterned GaAs(001) substrates with adjustable wavelengths. Interplay of the dot growth parameters, particularly growth temperature and Indium deposition amount, as well as the size of the initial template has been employed. With a 20 nm thick GaAs spacer layer grown between the regrowth interface and the quantum dot layer, uniform arrays of quantum dots have been achieved with emission wavelengths covering a spectral window ranging from 900 nm to 1200 nm. This has been achieved without risking the single dot occupancy per nucleation site measured to be $>$ 60{\%} for all of the investigated samples. To ensure better quality of dots, wafer cleanliness is monitored throughout the process. The dots thus show bright emission lines with no spectral jittering.   

Investigation of spatial correlation of type-II ZnTe quantum dots embedded in ZnCdSe barriers

Doped and undoped multilayered structures of ZnTe type-II quantum dots (QDs) embedded in a ZnCdSe matrix have been grown in order to investigate the formation of an intermediate band, lying within the ZnCdSe band gap, with the aim of absorbing photons with energies below ZnCdSe bandgap. These materials may be useful for intermediate bandgap solar cells. The reciprocal space map (RSM) of the ZnTe/ZnCdSe multilayer QD structure consisting of periodic superlattice peaks in the $q_x$ direction have been studied for two different ZnCdSe spacer thicknesses (d$_A$$\approx$3.5, d$_B$$\approx$1.5 nm). The ZnTe QDs give rise to diffuse scattering in RSM, which is found to be elongated in the $q_x$ direction for both samples indicating a vertical correlation of the QDs. From the widths of the diffuse maxima in the $q_z$ direction, we found that 16\% and 40\% of QDs are correlated vertically for d$_A$$\approx$3.5 nm and d$_B$$\approx$1.5 nm, respectively. With increasing correlation, the pairing probability of the dots increases, leading to a larger average QD size. This conclusion is supported by a smaller blue-shift (26 vs 36 meV) of the photoluminescence peak position with increasing excitation intensity, over five orders of magnitude, for the structure with narrower spacers.   

Characterization of quantum dot chains using transmission electron microscopy

We report on the growth and characterization of InGaAs self-assembled quantum dots which form into chains through an altered Stranski-Krastanov method. The methods we are using to study these quantum dot chains include imaging and chemical analysis using a transmission electron microscope (TEM). In order for the quantum dot chains to be characterized using the TEM, the samples must be cut and thinned to allow enough electrons to pass through the sample for our techniques. We are making cross-section and plan view cuts which allow us to get information about the chemical composition, indium segregation, size and spacing, contaminates and other aspects of the dots.