Molecular dimensions are obviously well beyond the resolution of standard or even state-of-the-art lithographic techniques. This fact means that, in order to fabricate true nanoscale molecular electronics circuits, the molecules themselves represent just one part of the game (1st cycle). Another aspect involves coming up with fabrication methodologies for nanoscale construction that do not rely on lithographic processing. The only obvious alternative is chemical assembly by chemical reaction at the nanoscale, and its use in electronic manufacture opens up a third set of issues, namely, which molecules and materials are consistent with both chemical assembly and electronic circuitry? How the energy can be transferred from the molecule to the nanomaterial (2 cycle)? Finally, it is necessary to interface whatever nanoscale architectures are fabricated with the outside world, indicating the need for some structure that can interface (or multiplex) large numbers of molecular electronics devices with small numbers of wires (3 cycle). This component of the molecular optoelectronic circuit represents serious and fundamental scientific challenges, to the extent that the collective task appears daunting indeed. As with any chemical problem, it is important to know what to make, and so we will first rationalize our approach to devices within the context of molecular surfaces. The three cycles (3C) summarizes the activity of the lab and illustrate in Figure 1.
Figure 1. The 3Cs principle of the lab.
Exploring the 3C principle may deliver a new technology and devices in different disciplines. As an example, we combine semiconducting nanowires (here we propose to use Si NWs) as the confined carrier (the interface) and molecular building blocks (photochromatic molecules, electronic molecules). This combination may address the aforementioned issues and possible energy scenarios between the molecules and the interface (such as light and charge transfer) may have happened. It's important to mention that Si NWs are an important semiconducting material with high chemical stability and can be prepared in large-area arrays in a controlled fashion. In addition, the ease of modification and compatibility of Si NWs with the prevalent integrated technology of silicon make Si NWs promising for construction of future nanosized devices such as logic gate systems (the hybrid device). In this case, the functionality of the hybrid device is determined mainly by the molecule. For example, redox active molecules can be useful for storing charges while fluorescent dyes and/or ion-activated molecules can be useful for logic gates. However, scientific challenges involved with this approach include schemes to convert efficiently the input signal (either optical or electrical) to an output signal, the role of the functional molecule and the molecular confined surface. To this end, an efficient energy transfer over the interface (nano distances), advanced surface characterization tools, and a fundamental understanding of the chemical reaction with the molecular functionality are all required. Specifically, to realize the 3C principle, four scientific challenging have to be overcome: (i) ensuring long lifetime of the generated charges, (ii) facilitating the energy (ΔФ) for holes and electrons through molecular transport channels in an appropriate way, and (iii) grafting correct molecules for such functionality on the interface with soft methods.
These objectives will be addressed within the 3C lab and will be accomplished through breakthrough methods that fabricate junctions to traverse carriers with “short” distances, grafting molecules, molecular characterization and device fabrication.
3.1 Molecules: We have developed a fully programmable nano-reactor that can maintain constant high temperatures (up to 500 °C) and pressures (up to 350 bars). In addition, the nano-reactor is also at a high resolution of time durations (in the scale of seconds), pressure steps (100 bars in 1 min), and temperature steps (100 °C in 1 min) to generate shock-waves. The extreme conditions are important to overcome the Van-Der-Waals interactions (while reserving the Si NWs) and to obtain a full molecular coverage regardless to the molecular steric effect (see Figure 2 as an example).
Figure 2. Grafting the passive molecules. Example grafting of passive molecules through the various Si-C bonds. Different shock wave conditions – last step, (such as temperatures, pressure, and reaction time), will be applied used to obtain a full molecular coverage for each molecule (R).
3.2 Interfaces: Si NWs: The Si NWs are realized with Chemical Vapor deposition (CVD), See Fig 3a. Here we can get any geometrical property (diameter and length) and doping profile (n or p-type).
3.3 Energy Transfer: We differentiate between two types of energies: Electrical and optical transfer. For electrical energy transfer (such as charge transfer) can be accomplished through the termination of dangling bonds via functional molecules (passive molecule) and molecular interface engineering of the molecular orbitals (HOMO/LUMO) with respect to the Si band edges for controlling the barrier height for electrons (ФBn) and holes (ФBp), so-called the active molecule, (see Figure 3b). For the optical its can be accomplished by (i) changing the dipole moment of the molecule after absorbing light, (ii) emitting light, (iii) charge transfer after light, (see Figure 4-right).
Figure 3. (a) Si NWs grown by CVD, (b) illustration of charge transfer by two layer molecular, first layer is passive and 2nd layer in active.
3.4 Devices: For each active molecule we design a special device accordingly. For example, three different devices are shown in Figure 4. Basically, the electronic and the optical active molecules will be integrated in FET, while magnetic molecules integrated in superconducting quantum interference device. For an electronic molecule, we used CoCp2 molecule with (Ei= 3.3 eV) i.e. give electron to Si. In this way the conductivity of the channel can be increased (in case of n-type Si NW). For a photocheomatic molecule, we used Azobenzen: the Vth of the 1D-FET can be tuned by the dipole moment of the molecule (from cis to trans).
Figure 4. (a) Electronic molecule based FET, (b) Photochormatic molecules based FET.
All the 3Cs are characterized by physical methods such as (i) X-Ray photoelectron spectroscopy (XPS), (ii) Photoelectron yield spectroscopy (PYS), and (iii) Kelvin probe force microscopy (KPFM).