PacSurf2014 Session NM-TuM: Nano Devices

Idea of utilizing individual molecules as the electronic components in future ultrahigh-density electronic devices has generated tremendous attention. I will explain recently developed understanding on the electrical transport characteristics through various types of molecular junctions on flat or flexible substrates [1-3]. In particular, obtaining transistor action from molecular orbital control has been the outstanding challenge of the field of molecular electronics nearly since its inception. In this talk, I will demonstrate a direct electrostatic modulation of orbitals in a molecular transistor configuration in electromigration nanogap [1] or in mechanically-controllable break junction (MCBJ) [2]. I will also demonstrate functional devices such as diodes or photoswitches at the molecular-scale on both rigid and flexible substrates [3]

I will also present a brief summary on general characteristics of the materials, device structures, and switching mechanisms used in polymer-based non-volatile memory devices. Strategies for performance enhancement, integration, and advanced architectures in these devices will be presented [4].

References:

[1] Nature 462, 1039 (2009); Adv. Mater. 23, 1583 (2011)-review

[1] Nano Lett. 13, 1822 (2013); Adv. Mater. 25, 4845 (2013)-review.

[3] Nature Nanotech. 7, 438 (2012); Adv. Funct. Mater. 24, 2472 (2014); Adv. Mater. in press (2014). DOI: 10.1002/adma.201306316

[4] Adv. Funct. Mater. 21, 2806 (2011)-review.

High-throughput nanogap formation is reported for simultaneously fabricating arrays of integrated nanogaps. Using this method, series-connected 10 nanogaps with symmetrical and asymmetrical shapes were integrated. The integration was achieved using electromigration (EM) induced by First, series-connected 10 Ni nanogaps having symmetrical shape were fabricated by electron-beam (EB) lithography and lift-off process. After performing the activation with final preset current I_S = 300 nA into the 10 nanogaps, the separation of the gaps was reduced to less than 10 nm. This tendency is quite similar to that of series connected 10 nanogaps having asymmetrical shape. Therefore, it is indicated that integration of nanogaps using activation method hardly depends on the shape of nanogap electrodes. Furthermore, activation method was also applied into 30 nanogaps connected in series, for the mass production of identical nanogaps. As a result, the distance between the Ni nanogap electrodes was totally and completely controlled by performing the activation. These results clearly suggest that the integrated nanogaps can be simultaneously fabricated by the activation procedure.

Single-walled carbon nanotubes have unique optical properties as a result of their one-dimensional structure. Not only do they exhibit strong polarization for both absorption and emission, large exciton binding energies allow for room-temperature excitonic luminescence. Furthermore, their emission is in the telecom-wavelengths and they can be directly synthesized on silicon substrates, providing new opportunities for nanoscale integrated photonics.

Here we discuss the use of individual single-walled carbon nanotubes for optical devices that could be integrated in silicon photonics. Their light emission properties can be controlled by coupling to silicon photonic structures such as photonic crystal microcavities [1,2] and microdisk resonators [3]. With the strong absorption polarization at the nanoscale, they allow for unconventional polarization conversion that results in giant circular dichroism [4]. More recently, we have found that excitons can dissociate spontaneously [5], enabling photodetection at low bias voltages. Ultimately, it should be possible to combine these results to achieve generation, manipulation, and detection of photons on a chip.

Work supported by SCOPE, KAKENHI, The Canon Foundation, The Asahi Glass Foundation, KDDI Foundation, and the Photon Frontier Network Program of MEXT, Japan. The devices were fabricated at the Center for Nano Lithography & Analysis at The University of Tokyo.

[1] R. Watahiki, T. Shimada, P. Zhao, S. Chiashi, S. Iwamoto, Y. Arakawa, S. Maruyama, Y. K. Kato, Appl. Phys. Lett. 101, 141124 (2012).

[2] R. Miura, S. Imamura, R. Ohta, A. Ishii, X. Liu, T. Shimada, S. Iwamoto, Y. Arakawa, Y. K. Kato, submitted.

[3] S. Imamura, R. Watahiki, R. Miura, T. Shimada, Y. K. Kato, Appl. Phys. Lett. 102, 161102 (2013).

[4] A. Yokoyama, M. Yoshida, A. Ishii, Y. K. Kato, Phys. Rev. X 4, 011005 (2014).

[5] Y. Kumamoto, M. Yoshida, A. Yokoyama, T. Shimada, Y. K. Kato, Phys. Rev. Lett. 112, 117401 (2014).

Singularity dictates a device function at the nanoscale. Dopants or impurities, structural defects, adsorbates, and stray charges can behave as a singularity in certain conditions, either promoting or deteriorating the device function, which frequently is the major concern in the implementation of nanoscale memories and sensors.

In a nanodevice adopting nanomaterials, the control over singular points in the nanomaterials can be regarded as a tuning process of the device property, which may open up a new possibility of its application to the molecular measurements. In the earlier part of this talk, an example of the property-tuned nanodevice for molecular measurements will be discussed after a short introduction of visualizing the singularities in a nanodevice based on the nanomaterials.

An extreme case of the singularity can be found in a nanogap device which has two electrodes separated by a few to a few tens of nanometers. In the later part of this talk, application of nanogap devices in the electric/electrochemical (bio) molecular detection will be discussed with our recent experimental results along with the way of their simple lab-scale mass production.

Therefore, we investigated the band structure and the Fermi level pinning at clean and well-ordered sidewall surfaces of zincblende (ZB)-wurtzite (WZ) GaAs nanowires by scanning tunneling spectroscopy and density functional theory calculations. The WZ-ZB phase transition in GaAs nanowires introduces p‑i junctions at the sidewall surfaces. This is caused by the presence of numerous steps, which induce a Fermi level pinning at different energies on the non-polar WZ and ZB sidewall facets.