Our recent research work mainly focuses on combined molecular engineering and advanced nano-lithographic techniques for fabrication of functional devices and detectors. Particularly, attempts have been made on manipulation and detection of individual DNA molecules. In that project, we have rudimentary, but substantial progresses, and there are on-going research activities along this line. In addition, we continue to work on topics that are related to the transport properties of nano-electronics, including nanowire devices and lithographically made silicon devices. In the following, we present high lights of our recent research works.

 

 

Scaling Analysis of Magnetic Field Tuned Phase Transitions in One-Dimensional Josephson Junction Arrays
( Physical Review Letters , 87, 186804, (2001))

We have studied experimentally the magnetic field induced superconductor-insulator quantum phase transition in one-dimensional arrays of small Josephson junctions. It is found that the critical magnetic field that separates the two phases corresponds to the onset of Coulomb blockade of Cooper pairs tunneling in the current-voltage characteristics. The resistance data are analyzed in the context of the superfluid-insulator transition in one dimension, and a finite temperature scaling analysis is performed to extract the critical exponents. The dynamical exponents z are determined to be close to 1, and the correlation length exponents n are found to be approximately 0.3 and 0.45 in the two groups of measured samples.

The SEM image of an 1D JJ array. The overlappingareas between “T”-shaped islands are the tunnel junctions. The scale bar at the bottom of the image is 1 m m.	The R 0 ( T ) as a function of the filling number f. At T > T cr , the array displays an f -tuned SI transition, whereas at low temperatures the R 0 ( T ) curves in the S side level off or rise.

 

 

Suppression of superconductivity by spin imbalance in ferromagnetic-
superconductor-ferromagnetic single electron transistors
( Physical Review Letters , 88, 047004 (2002))

We present here an experimental demonstration for suppression of superconductivity by spin imbalance. This effect is manifested under spin-polarized quasiparticle current injection in ferromagnet-superconductor-ferromagnet (Co/Al/Co) single electron transistors. The measured superconducting gap as a function of magnetic field reveals a dramatic decrease when the magnetizations of the two leads are misaligned. The effect of suppression increases with increasing source-drain voltage. A comparison with numerical calculations for single electron transistor in sequential tunneling regime is performed. The imbalance of spins is a nonequilibrium process. For this process to be effective, a sufficiently long spin relaxation time and a short energy relaxation time are required. Various factors that may affect this process are considered. This method may render it applicable to control superconductivity at low temperatures within low fields.

AFM and (b) magnetic force microscope (MFM) images of a sample. Because of a two-angle evaporation technique used for fabrication of the samples, there are redundant, electrically unconnected structures aside the measured device. The inset in (a) illustrates the cross section of the island and junctions.

 

 

Single-Electron Transistors and Memory Cells with Au Colloidal Islands
( Applied Physics Letters , 81, 4595 (2002))

In this study, single-electron transistors and memory cells with Au colloidal islands linked by C 60 derivatives have been fabricated by hybridization of top-down advanced electron-beam lithography and bottom-up nanophased-material synthesis techniques. Low-temperature transport measurements exhibit clear Coulomb-blockade-type current-voltage characteristics and hysteretic-type gate-modulated current. The hysteresis is attributed to the presence of electrically isolated charge-storage islands. With the guidance provided by Monte Carlo simulation, we propose a circuit model and give an estimate of the sample parameters

An SEM image of the measured device; the gate electrode is not shown. The inset shows the suspended Au leads before attachment of Au particles; the scale bar is 150 nm.	A three-dimensional plot of I sd vs V g and V b . The curves are measured at T =4.2 K. Note that the modulation is irregular, but the curves are reproducible, and a Coulomb-blockade region is clearly seen.

 

 

Fabrication of Two-Dimensional Arrays of CdSe Pillars Using E-Beam Lithography and Electrochemical Deposition
( Advanced Materials , 15, 49 (2003))

Two-dimensional arrays of high refractive index structures can be fabricated using a combination of e-beam lithography for pattern definition and electrochemical deposition for structure formation. The potential of this method is demonstrated for CdSe, where nanopillars, walls, and crosses are prepared. Such arrays have potential in optical device applications such as photonic crystals and waveguides.

SEM image of (a) mushroom-like pillars and (b) a continuous CdSe film formed on top of the pillars)	SEM images of (a) an array of CdSe walls, where the length of the walls is about 20 m m and cross-shaped walls, where the length is 2 m m and the height is about 400nm	Procedures for e-beam lithography. (a) spin-coating of bilayer e-beam resist (b) Exposure to focused Gaussian shaped electron beam (c) Development of the exposed pattern (d) oxygen reactive ion bean cleaning (e) Chemical electrodeposition of CdSe (f) Resist-removal (lift-off process) in acetone bath.

 

 

Controlled Placement and Electrical Contact Properties of Individual Multi-walled Carbon Nanotubes on Patterned Silicon Wafer
( Applied Physics Letters , 84, 984 (2004))

A scheme that allows on-chip growth of multiwalled carbon nanotubes at designed locations is demonstrated. The nanotubes were grown by thermal chemical vapor deposition and were contacted to nanoscaled Cr electrodes fabricated by standard e-beam lithography techniques. The contacts were found to be Ohmic with resistance values on the order of 10 3 V at room temperature. Remarkably, the contacts showed weak temperature dependence down to 40 mK and were insensitive to the magnetic field up to 5 T.

An SEM image of one of the measured MWNTs. The tube was grown from an iron catalyst ~not shown in this image! and was connected to nine Cr electrodes.	The asymptote resistance at 70 mK as a function of the perpendicularly applied field. Both two-probe and four-probe magnetoresistance decreases with increasing field. A comparison between the two curves indicated that the contact resistances are unaffected by the applied fields. The inset shows four-probe magnetoresistance of a separate tube.

 

 

Positioning of extended individual DNA molecules on electrodes by non-uniform
AC electric fields
( Nanotechnology 16 , 2738–2742 (2005) )

Recent developments in the analysis and the application of DNA often require the stretching of individual DNA molecules to specific surfaces. We propose and demonstrate a method for the positioning of unmodified extended DNA molecules. A local microscopic circular flow is created by a non-uniform AC field and utilized to stretch the λ -DNA on a gold surface or between gold electrodes. The electrical-field amplitude and frequency responses of DNA motion are studied. The method can be applied to position the DNA with accuracy on a microscopic scale while requiring no modification of the DNA for terminal binding. With a diluted DNA solution, the number of DNA molecules across the electrodes is controllable and the positioning of a single extended DNA across electrodes is achievable.

Schematic diagram illustrating the motion of DNA in an AC field. (a) A cyclic motion results from electro-osmosis flow (EOF). EOF draws DNA near the inner edges inward, and pushes them outward to the outer edges. (b) Four successive steps, a–d, show how a DNA molecule may anchor and extend in corresponding to the cyclic motion. This circular movement is mainly developed in the x – z plane.

Images showing the results of positioning the DNA across electrodes. (a) Dark field image from light microscope shows the arrangement of the electrodes. The AC electric circuit is illustrated. The width of the narrowest electrodes is 400 nm. The bright areas are gold electrodes on top of the glass substrate which appears as dark background. The scale bar is 10 m m. (b) Fluorescence image shows two DNA molecules positioned on each group of electrodes. The applied voltage is 2 Vp ? p with a frequency of 200 Hz. (c) AFM image shows the area marked with the dotted rectangle in (b). The two arrows indicate the two DNA molecules across electrodes. The scale bar is 1 m m.

 

 

DNA as an Electron-Beam Sensitive Reagent for Nano-Patterning
( Advanced Materials , 18 , 1517–1520 ( 2006 ))

In this study, we propose and demonstrate a new application using DNA as an e-beam sensitive reagent for patterning. The technique allows direct electron-beam patterning of oligonucleotides. To this end, thiolated single-strand DNA was bombarded using a focused electron beam, resulting in the inhibition of hybridization to complementary strands. The degree o f inhibition as a function of the exposure dose was studied using both fluorescence-probe and Au-nano-particle labeling. Finally, for demonstration purposes gold nano-particles were used as markers to produce nano-scaled patterns. The results of which are presented in this paper. This technique has potential applications in the fabrication of DNA-based nano-structures.

Sample preparation procedures. a) Application of HS-20T solution to a gold surface. b) Immobilization of HS-20T oligonucleotides through sulfur–gold interaction. c) Electron-beam patterning on the oligonucleotide thin film. d) Hybridization of Hex-20A with HS-20T.

G old nano- particle pattern produced by utilizing DNA molecular layer as an e-beam sensitive reagent. In this approach, thiolated single-strand DNA was bombarded using a focused electron beam, resulting in the inhibition of hybridization to complementary strands , and then gold nano-particles were used as markers to reveal nano-scaled patterns . This technique has potential applications in the fabrication of DNA-based nano-structures .

 

 

Generation of nano-scaled DNA patterns through electro-beam induced charge trapping
( Nanotechnology 17 , 1–5 (2006) )

In this study, distinct regions of trapped charges on glass substrates created by electron beam bombardment were utilized to attract and to immobilize DNA molecules. The negatively charged DNA molecules were attracted by the positive charge layer beneath the substrate surface resulting from escape of secondary electrons. With this mechanism, we demonstrated high-precision patterning of unmodified DNA molecules, independent of the length, sequence, and number of DNA strands, and with an attachment to the glass surface strong enough to withstand vigorous washing with water. DNA patterns with the line width of 50 nm were achieved.

		(a) Energy dependence of secondary electron yield quoted from [ 8 ]. (b) Equipotential curves above the substrate surface. The zero-potential curve (in yellow) divides the potential space into two regions: an inner positive potential region (red curves) and an outer negative potential region (blue curves).
(a)–(d) SEM images of uranyl acetate stained DNA patterns. The dark lines are formed by DNA molecules at (a) 500, b) 100 and (c) 50 nm line widths. (d) The characters ‘SINICA' formed by DNA at 400 nm line width. (e) AFM image of the same sample shown in (d).

 

 

Coupled single electron transistors as a differential voltage amplifier
( New Journal of Physics , 8, 300 (2006) )

We have investigated a possible application of single electron transistor (SET) devices for use as a differential voltage amplifier. The device consists of a pair of box-SET and probe-SET coupled with each other through a tunnel junction, with the gate electrodes of the two SETs acting as differential signal inputs. The voltage across the probe-SET at a fixed bias current provides information about the charge states of both the probe-SET and the box-SET, which was confirmed by simulations based on the orthodox theory of single-electron tunneling. When operated as a differential amplifier, the output probe-SET voltage signal was measured as a function of the two gate input signals. While the output signal was found to be proportional to the difference in the two input signals, it remained unchanged for input signals of the same amplitude (referred to as the common mode signal), and the common mode rejection ratio was found to be 27.5dB.

SEM image of a measured sample. The junctions areas are marked by white parallelograms and the two control gates are located outside the image.	( left ) Intensity plot of V P as a function of gate voltages V bg and V pg measured at I p =0.43nA. Bright regions indicate high V P values. ( right ) The same plot as in ( left ) but made by simulation. The dashed line and dotted line indicate the applied gate voltages for common-mode and differential-mode input signals, respectively.

 

 

Fabrication of One-Dimensional Au-Particle Electronics with DNA-mediated Charge Trapping Technique ( Advanced Functional Materials , 17, 3182 (2007) )

We report a unique approach for producing nano-scaled one-dimensional (1D) gold-particle electronic devices. In this approach, a focused electron beam was first utilized to generate a positive charge layer on a SiO 2 surface. Biotinated DNA molecules attracted by these positive charges were subsequently used to grasp Au-nanoparticles revealing the e-beam exposure single-line patterns. Due to repulsive force between Au colloidal particles, the particles in the single-line patterns were, to a large extent, orderly separated. We further develop a simple method to bridge the particles to form conductive nanowires of high or low wire resistance. While low resistance wires showed linear current-voltage characteristics with a high maximum allowed current density, the high resistance wires exhibited charging effect with clear Coulomb oscillation behavior at low temperatures. This demonstrates that the technique is capable of producing interconnects as well as single-electron-transistors, and opens up possibilities for fabrication of integrated circuits.

(a~g) SEM images of Au-nanoparticle single-line patterns. The four writings of "DNA" in (a) were made utilizing e-beam exposure line doses of, from top-right to bottom-left, 1.0, 1.5, 2.5, and 4.0 nC/cm, respectively, and the patterns in (b~g) were made with a line dose of 2.5 n C/cm. For all patterns, the center-to-center distance between the beam pulses was set to 13.6nm.	Top: SEM images of one dimension Au particle array and nanowires: (a) before bridging cycle, (b)~(c) after one bridging cycle and (d)~(f) after two bridging cycles. (g) Au particle nanowire attached between a pair of electrodes for electrical measurements . Bottom: Gate-voltage modulations of source-drain current at ramping bias voltages. The curves were taken at 6K. The zero-current Coulomb parallelogram and Coulomb oscillations are signatures of SET.

 

 

Polymer-based photonic crystals
fabricated with single-step electron beam lithography (Advanced Materials , 19, 3052 ( 2007) )

We present a simple, versatile technique for the fabrication of quasi three-dimensional suspended polymer photonic crystals (PCs) and three-dimensional multilayer polymer photonic crystals. For quasi-3D PC slabs, photonic band gap in the visible light region was evidenced from optical microscopic observation and transmission spectrum measurement. In addition, slabs with photonic band gap in both TE-like and TM-like modes for the telecommunication wavelength region were designed and fabricated. This unprecedented fabrication method utilizes only a single-step electron beam lithography process, and thus overcomes difficulties encountered by existing 3D PC techniques. This is a step forward to the realization of multifunctional PC integrated circuits. (with supplemental material)

(left) SEM image of a suspended PMMA quasi-3D PC slab with a hexagonal array of air holes. The hole radius is 260nm, and the lattice constant is 800nm. (right) Measured transmission spectrum. The calculated gaps are 454~484nm and 491~508nm.
(left) Cross-section view of a multilayer PMMA/LOR line-array structure. The structure is made using a single exposure step and layer–by-layer development processes. The PMMA and LOR layers are 200nm- and 500nm-thick, respectively. (right) SEM image of a line defect created by adjusting the exposure dose.

 

 

Cyclotron Localization in a sub - 10nm Silicon Quantum Dot Single Electron Transistor ( Applied Physics Letters , 90, 032106 (2007) )

W e have fabricated and measured a lateral Si-SET consisting of a succession of a big island and small quantum dot s. In th is device , small Coulomb oscillation wiggles, due to the big island, acted as a scale to reveal shifts in peaks of Coulomb oscillation envelopes, due to the small quantum dots, in the presence of a magnetic field. The observe d shift s in peak position are analyzed in the context of field-induced Landau level shift in dots with a soft-wall confinement potential. Furthermore, the current peak was suppressed for fields beyond a threshold value. A n explanation based on cyclotron localization at non-interacting L andau levels of the small quantum dot s is presented.

	The oscillatory current modulation in gate voltage at B=0 (black curve) and B=5T (red curve) measured at 70mK for the device shown in the top-right inset. The device contains a big island connected to leads via small dots present in the nano-constrictions. The top-left inset presents IV b curves at ramping V g clearly marking the C oulomb blockade diamond ; a closer look reveals fine wiggles arising from Coulomb oscillation in the big island.
	(top) Dynamics of 5 current peaks in magnetic fie l d at a bias voltage of 1.8 mV. The curves shift with field from -5T at the bottom to +5T at the top. The current peak s at zero magnetic field are clearly suppressed as seen from peak P3. Refer to the main text for the description of the origin of the shift and suppression of the current peak. The inset presents enlarged view of P5 describing the evolution of current peak position with the applied field s of 0 (black curve), 2.5T (green curve) and 5T (red curve) . Note also the small wiggles show no shift in the field. The peak position shift in (bottom) as a function of magnetic field for the 5 peaks are extracted from the raw data shown in (top) for further analysis. Peaks 2 and 3 show clear splitting in the presence of field, and the peaks shift in opposite directions .
	A comparison between (left) the theoretical eigen energy spectrum in the presence of external field and (right) the experimental current peak position data . The zero field regions have multifold degeneracy and the beginning of our comparison with the theoretical curves from B>2 T and ω c /ω o > 2 is justified.

 

 

Control and detection of organosilane polarization on nanowire field-effect-transistors ( Nano Letters , 2007)

We demonstrated control and detection of UV-induced 3-Aminopropyltriethoxysilane (APTES) polarization using silicon nanowire field-effect-transistors made by top-down lithograph technology. Electric dipole moment in APTES films induced by UV-illumination was shown to produce negative effective charges. When individual dipoles were aligned with an externally applied electric field, the collective polarization can prevail over the UV-induced charges in the wires and give rise to an abnormal resistance enhancement in n -type wires. Real-time detection of hybridization of 15-mer poly-T/poly-A DNA molecules was performed, and amount of hybridization induced charges in the silicon wire was estimated. Based on these results, detection sensitivity of the wire sensors was discussed.

	(a) SEM image of the SOI-based Si-wire FET device. The width, length and thickness of the wire are 100nm, 3 m m and 30 nm, respectively, and the wire is covered by a 10-nm thick thermally oxidized SiO 2 layer. The schematic drawings illustrate (b) APTES surface modification and (c) detection of DNA-hybridization .
	a-c) Schematic illustration of APTES polarization on a n -type SiNW: (a) random orientation of molecules with internal dipoles; (b) because of the internal field, a high E-field helps to align APTES; (c) UV-illumination strengthens the internal dipoles. (d) UV-responses of an n-type wire before (top panel) and after (bottom panel) subjecting to an E-field as shown in (b). In the former, the resistance decreases under UV-illumination, as expected. In the latter molecular dipole field prevails, and the device shows a reversed response.

easured nanowire resistance in response to the injection of ssDNA molecules. Prior to the measurement the wires were modified with 15T ssDNA. (a) 15C ssDNA (cyan shade) and buffer (gray shade) solution were added at t=150 and 650 sec, respectively, and the wire showed no change in resistance. At t=800 sec, 15A ssDNA (yellow shade) was added and an abrupt increase in the resistance was observed. The two curves are for two SiNWs measured simultaneously. (b) The resistance of two p-type wires measured simultaneously as a function of gate voltage before addition of 15A ssDNA (dotted curve) and after hybridization (solid curve).

 

 

TOP