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The interaction of deposited metals with monolayer films is critical to the understanding of, and ultimate utility of, the emerging arena of molecular electronics. We present the results of a thorough study of the interaction of vapor-deposited Au and Ag on alkane films attached to Si substrates. Two distinct categories of films are studied:  C18 films formed from the hydrosilation reaction of octadecyl trichlorosilane with thin SiO2 layers and C18 films formed from the direct attachment of functionalized alkanes with hydrogen-terminated Si. Two direct attachment chemistries were studied:  (Si)3−Si−O−C linkages of 1-octadecanol and octadecanal on H-terminated Si(111) and (Si)3−Si−C linkages of 1-octadecene on H-terminated Si(111). The reactivity of the films was studied with p-polarized backside reflection absorption spectroscopy (pb-RAIRS), sputter depth profiling X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), atomic force microscopy (AFM), and device electrical tests. Independent of direct attachment chemistry, we report the remarkable observation that deposition of Au results in the displacement of the molecular film from the Si interface. In contrast, the directly attached molecular films are robust toward the deposition of Ag. For both metals, the C18 films formed by hydrosilation reactions on SiO2 remain at the interface. The results of monolayer stability with metal are linked to reactions between the metal and substrate. The displacement of the films by Au is attributed to Au insertion in the Si backbonds, in a reaction analogous to silicide formation. The results demonstrate that one must fully take into account the reactivity of the entire system, including substrates, molecular functional groups, and metal electrodes, when considering the robustness of molecules in metal junctions.

We report the fabrication of molecular electronic test structures consisting of Au-molecule-Si junctions by first forming omega-functionalized self-assembled monolayers on ultrasmooth Au on a flexible substrate and subsequently bonding to Si(111) with flip-chip lamination by using nanotransfer printing (nTP). Infrared spectroscopy (IRS), spectroscopic ellipsometry (SE), water contact angle (CA), and X-ray photoelectron spectroscopy (XPS) verified the monolayers self-assembled on ultrasmooth Au were dense, relatively defect-free, and the -COOH was exposed to the surface. The acid terminated monolayers were then reacted with a H-terminated Si(111) surface using moderate applied pressures to facilitate the interfacial reaction. After molecular junction formation, the monolayers were characterized with p-polarized backside reflection absorption infrared spectroscopy (pb-RAIRS) and electrical current-voltage measurements. The monolayer quality remains largely unchanged after lamination to the Si(111) surface, with the exception of changes in the COOH and Si-O vibrations indicating chemical bonding. Both vibrational and electrical data indicate that electrical contact to the monolayer is formed while preserving the integrity of the molecules without metal filaments. This approach provides a facile means to fabricate high-quality molecular junctions consisting of dense monolayers chemically bonded to metal and silicon electrodes.

Semiconductor-molecule-metal junctions consisting of alkanethiol monolayers self-assembled on both p(+) and n(-) type highly doped Si(111) wires contacted with a 10 µm Au wire in a crossed-wire geometry are examined. Low temperature transport measurements reveal that molecule-induced semiconductor interface states control charge transport across these systems. Inelastic electron tunneling spectroscopy also highlights the strong contribution of the induced interface states to the observed charge transport.

Molecular electronics has drawn significant attention for nanoelectronic and sensing applications. A hybrid technology where molecular devices are integrated with traditional semiconductor microelectronics is a particularly promising approach for these applications. Key challenges in this area include developing devices in which the molecular integrity is preserved, developing in situ characterization techniques to probe the molecules within the completed devices, and determining the physical processes that influence carrier transport. In this study, we present the first experimental report of inelastic electron tunneling spectroscopy of integrated metal-molecule-silicon devices with molecules assembled directly to silicon contacts. The results provide direct experimental confirmation that the chemical integrity of the monolayer is preserved and that the molecules play a direct role in electronic conduction through the devices. Spectra obtained under varying measurement conditions show differences related to the silicon electrode, which can provide valuable information about the physics influencing carrier transport in these molecule/Si hybrid devices.

In this work, we establish the potential of a UV-promoted direct attachment of alkanes with alcohol and thiol linkers to the silicon (100) surfaces for use in molecular electronic devices with increased potential for integration with existing CMOS technologies. Characterization of the self-assembled monolayers via Fourier transform infrared spectroscopy, spectroscopic ellipsometry, and X-ray photoemission spectroscopy shows that the films assembled on the Si (100) are comparable in quality, aliphatic monolayer coverage, and extent of substrate oxidation to those assembled on the more extensively studied Si (111) crystal face. Simple Si (100)-based electronic devices fabricated with the monolayers exhibited molecule-dependent electrical characteristics. These data highlight the effectiveness of the assembly on Si (100), the ability to fabricate enclosed Si (100)-based molecular devices, and the potential for the future integration of these devices with more conventional technologies.

In order to realize molecular electronic devices, molecules with electrically interesting behavior must be identified. One molecule that has potential for use in devices is an oligo(phenylene ethynylene) (OPE) molecule with nitro sidegroup(s). These "nitro" molecules have been reported to show electrical switching with memory behavior, as well as negative differential resistance (NDR). However, different research groups testing the nitro molecules in different test beds have observed different electrical behaviors. In this work, we assembled two different nitro monolayers: one completely composed of nitro molecules and the second a mixed matrix where nitro molecules were separated by dodecanethiol molecules. We used scanning tunneling microscopy to image each of the monolayers and observed that the nitro molecules were effectively inserted into the ordered dodecanethiol monolayer. We tested the electrical behavior of the pure monolayer, as well as the mixed monolayer, in our nanowell test device. The nanowell devices were fabricated on micron-size gold lines patterned on oxide-coated silicon wafers. The gold lines were covered with a silicon dioxide layer, through which a nanometer size well was milled. This nanowell device was filled with a self-assembling monolayer of organic molecules, and capped with titanium and gold. The nanowell electrical results showed switching with memory for the pure nitro monolayer, but not for the mixed monolayer. This switching behavior consisted of a molecule starting in a high conductivity state and switching to a low conductivity state upon application of a threshold voltage. The high conductivity state could only be returned by application of an opposite threshold voltage.

In recent years many advances have been made in the development of molecular scale devices. Experimental data shows that these devices have potential for use in both memory and logic. This paper describes the challenges faced in building crossbar array based molecular memory, and develops a methodology to optimize molecular scale architectures based on experimental device data taken at room temperature. In particular, we discuss reading and writing such memory using CMOS and compiling a solution for easily reading device conductivity states (typically characterized by very small currents). Additionally, a metric is derived to determine the voltages for writing to the crossbar array. Simulation results, incorporating experimental device data, are presented using Cadence Spectre.