Optical effects on electronic transport through semiconductor nanoscrystals and single molecules
We are studying the effect of light on electron transport through single quantum dots. The quantum dot systems under examination include lead chalcogenide (PbS, PbSe) nanocrystals and photochromic dithienylethene molecules. We fabricate electrodes with a nanometer-scale gap and assemble the quantum dots of interest into this gap to form a single-electron transistor. In the semiconductor nanocrystals, we are investigating how light affects electron tunneling through the discrete energy levels of the quantum dot. In the photochromic molecules, we are seeking to measure the conductance of a single molecule as light switches it between its two different photo-isomerized states.
Eugenia Tam, Joshua Parks, and Dan Ralph
In our work, we combine previously established techniques  for contacting quantum dots in electromigrated junctions together with optical measurements to investigate the opto-electronic properties of PbS and PbSe nanocrystals and the conductance of single photochromic molecules.
The PbS nanocrystals currently used in our measurements are synthesized by the Wise group in the Department of Applied & Engineering Physics at Cornell . The particles are approximately 7 nm in diameter (Fig. 1(a)) and capped with oleic acid. Their absorption peak is in the infrared, at around 1600 nm. We are working to optimize the yield of single particles contacted between our nanoscale electrodes and to couple laser radiation to the device.
Our photochromic molecules are synthesized by the Abruņa group in the Department of Chemistry at Cornell. In solution, these molecules have been shown to switch from an open state to a closed (conjugated) state under exposure to light at about 320 nm and to switch back under irradiation at about 520 nm . The conductance of the molecule changes by orders of magnitude when switched from one state to the other . The molecules we are investigating (Fig. 2(a)) consist of a photochromic dithienylethene (DTE) unit with pyridine (Py) endgroups for a strong bond to gold or platinum electrodes. Fig. 2(b) shows another similar molecule we are working on that has a phenyl bridge between the switching unit and the pyridine endgroups.
We fabricate the devices (Fig. 1(b)) on oxidized silicon wafers and pattern the gold contacts and aluminum/aluminum oxide gates by photolithography. We use electron-beam lithography to define wires about 100 nm in width at the narrowest point. The wires are evaporated from either gold or platinum to a thickness of about 16 nm. We then break the wires using electromigration to form nanometer-scale gaps . The nanocrystals are incorporated into the gaps by dipping the chip in a solution of the nanocrystals in an organic solvent such as hexane. Devices with photochromic molecules are prepared by depositing about 20 ĩL of solution of the molecules in solvent onto the chip and blowing off the excess with nitrogen gas.
Our measurements are performed at room temperature and at liquid helium temperatures in cryostats designed to couple light onto the nanoscale junctions. For the nanocrystal measurements, we focus light from an argon-krypton laser onto the nanocrystal junction through a microscope and locate the precise location with piezoelectric-controlled deflection mirrors and reflection measurements. At low temperatures, from the conductance data, we are able to distinguish between bare junctions and junctions with a small number of dots trapped within the gap (Fig. 3). For the photochromic molecules, we perform our measurements in the aforementioned system and also with a halogen-deuterium broadband light source. In addition, we are developing a modified scanning tunneling microscope system  with the Abruņa group to study the conductance of single photochromic molecules using junctions immersed in solution.
Our investigations of the optoelectronic properties of semiconductor quantum dots and the conductance of photochromic molecules is ongoing. The results will be of interest for understanding the fundamental physics of these systems and should provide guidance for potential applications in nanoscale electronics.
Figure 1: (a) Scanning electron microscope image of PbS nanocrystals assembled into a nanometer-scale gap between electrodes. (b) Diagram of device geometry (not to scale).
Figure 2: Optical absorbance spectra of dithienylethene-based molecules (inset) in solution, showing optically-induced reversible switching. The molecules start initially in the open state (dotted line), are irradiated at 320 nm for 15 minutes (dashed line), and subsequently are irradiated at 520 nm for 1 hour (solid line). (b) Molecular diagram of a dithienylethene unit with phenyl bridges to pyridine endgroups.
Figure 3: Plot of conductance of a PbS nanocrystal sample as a function of bias voltage and gate voltage. The dark regions of current suppression are due to the charging energy required for sequential tunneling conduction through individual quantum dots. The junction from which this data were measured may have contained a single dot or a small number in parallel.
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Last updated: 19-June-2010