Linda Kunardi | National University of Singapore (original) (raw)

Papers by Linda Kunardi

Research paper thumbnail of Metal organic interfaces at the nanoscale

Nanotechnology, 2004

In this work, we present an investigation of the Ag-PPP (polyparaphenylene) interface using balli... more In this work, we present an investigation of the Ag-PPP (polyparaphenylene) interface using ballistic electron emission microscopy. Our work is the first successful application of the BEEM technique to metal-organic interfaces. We observe nanometer scale injection inhomogeneities. They have an electronic origin, since we find corresponding Schottky barrier variations. We also determine the transmission function of Ag-PPP interface and find that it agrees qualitatively with the theoretical calculations for a metal-phenyl ring interface. We conclude that charge transport across inhomogeneous barriers needs to be considered for understanding electronic transport across metal-organic interfaces and organic device characteristics.

Research paper thumbnail of Charge transport across metal molecule interfaces probed by BEEM

Journal of Physics: Conference Series, 2007

We use Ballistic Electron Emission Microscopy (BEEM) technique to determine directly the Schottky... more We use Ballistic Electron Emission Microscopy (BEEM) technique to determine directly the Schottky barrier distribution over silver /H-T 3 -(CH 2 ) 4 -HS (abbreviated as T3C4SH) self assembled monolayer interface area with nanometer scale spatial resolution. The selfassembled monolayer is absorbed on template stripped gold. BEEM images show spatially non-uniform carrier injection. A Wentzel Kramel Brillouin (WKB) calculation is performed and compared with BEEM spectra. The results show that the measured currents are four orders of magnitude larger than the direct tunnelling contribution, indicating molecular levels being accessed. To further substantiate the findings, characterization by STM distance versus potential spectroscopy is carried out to determine injection barriers at the interface. The results from these two techniques are compared and the implications of which will be discussed.

Research paper thumbnail of Spectroscopy and imaging of metal–organic interfaces using BEEM

Applied Surface Science, 2006

Charge injection from metal electrodes to organics is a subject of intense scientific investigati... more Charge injection from metal electrodes to organics is a subject of intense scientific investigation for organic electronics. Ballistic electron emission microscopy (BEEM) enables spectroscopy and imaging of buried interfaces with nanometer resolution. Spatial non-uniformity of carrier injection is observed for both Ag-PPP (poly-paraphenylene) and Ag-MEHPPV (poly-2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene) interfaces. BEEM current images are found to correlate only marginally with the surface topography of the Ag film. #

Research paper thumbnail of Switching in organic devices caused by nanoscale Schottky barrier patches

Chemical Physics, 2005

We have identified a possible electronic origin of metal filaments, invoked to explain the switch... more We have identified a possible electronic origin of metal filaments, invoked to explain the switching behavior of organic devices. Interfaces of two representative organics polyparaphenylene ͑PPP͒ and poly͑2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene͒ with Ag are investigated using ballistic emission microscopy. Nanometer scale spatial nonuniformity of carrier injection is observed in ballistic electron emission microscopy images of both interfaces. The measured Schottky barrier ͑SB͒ appears to be consistent with metal states tailing into the gap of the PPP. We find that the SB values exhibit a distribution, even for the diodes with low ideality factors. The implications of this distribution on the measured physical properties of the diode are discussed, in light of work on devices of similar geometry, published in the literature. We also demonstrate that patches of low SB are likely to nucleate current filaments which can cause local ionization and are reported to be responsible for the switching behavior observed in metal-organic, metal-CuS and Ag-AgSe structures.

Research paper thumbnail of Ballistic electron microscopy of a metal molecule interface

We present studies on a Ag/ HS–(CH2)4–T3–H (T3C4SH)/ Au diode using nanometer scale resolution, b... more We present studies on a Ag/ HS–(CH2)4–T3–H (T3C4SH)/ Au diode using nanometer scale resolution, ballistic electron emission microscopy (BEEM). Images show spatially non-uniform carrier injection. A WKB calculation is carried out and compared with the experimental data. The results indicate that molecular levels are being accessed in the BEEM experiment, since the measured currents are larger than purely tunneling contribution. Our results are consistent with previously published results on a similar molecule [ 1]. Physical origins of the non-uniform carrier injection and its implications are discussed.

Research paper thumbnail of Ballistic emission spectroscopy and imaging of a buried metal/organic interface

Applied Physics Letters, 2005

The silver-p-phenylene (Ag-PPP) interface is investigated using ballistic electron emission micro... more The silver-p-phenylene (Ag-PPP) interface is investigated using ballistic electron emission microscopy (BEEM). Multiple injection barriers and spatial nonuniformity of carrier injection over nanometer length scales are observed. No unique injection barrier is found. Physical reasons for these features are discussed. BEEM current images and the surface topography of the silver film are uncorrelated.

Research paper thumbnail of Possible transition from space-charge-limited to injection-limited conduction in poly(3-hexylthiophene) thin films

Applied Surface Science, 2006

Two-terminal thin films of poly(3-hexylthiophene) (P3HT) with a wide electrode separation (150 mm... more Two-terminal thin films of poly(3-hexylthiophene) (P3HT) with a wide electrode separation (150 mm) has been studied using current-voltage characteristics at different temperatures. Space-charge-limited conduction (SCLC) with high injection barriers (1.3 eV) has been observed at all temperatures in the low electric field regime. A possible transition from SCLC to injection-limited conduction (ILC) is reported. The experimental results have been compared with the disorder-controlled injection model proposed by Arkhipov et al.

Research paper thumbnail of Erratum: Switching in organic devices caused by nanoscale Schottky barrier patches

Research paper thumbnail of Metal organic interfaces at the nanoscale

Nanotechnology, 2004

In this work, we present an investigation of the Ag-PPP (polyparaphenylene) interface using balli... more In this work, we present an investigation of the Ag-PPP (polyparaphenylene) interface using ballistic electron emission microscopy. Our work is the first successful application of the BEEM technique to metal-organic interfaces. We observe nanometer scale injection inhomogeneities. They have an electronic origin, since we find corresponding Schottky barrier variations. We also determine the transmission function of Ag-PPP interface and find that it agrees qualitatively with the theoretical calculations for a metal-phenyl ring interface. We conclude that charge transport across inhomogeneous barriers needs to be considered for understanding electronic transport across metal-organic interfaces and organic device characteristics.

Research paper thumbnail of Charge transport across metal molecule interfaces probed by BEEM

Journal of Physics: Conference Series, 2007

We use Ballistic Electron Emission Microscopy (BEEM) technique to determine directly the Schottky... more We use Ballistic Electron Emission Microscopy (BEEM) technique to determine directly the Schottky barrier distribution over silver /H-T 3 -(CH 2 ) 4 -HS (abbreviated as T3C4SH) self assembled monolayer interface area with nanometer scale spatial resolution. The selfassembled monolayer is absorbed on template stripped gold. BEEM images show spatially non-uniform carrier injection. A Wentzel Kramel Brillouin (WKB) calculation is performed and compared with BEEM spectra. The results show that the measured currents are four orders of magnitude larger than the direct tunnelling contribution, indicating molecular levels being accessed. To further substantiate the findings, characterization by STM distance versus potential spectroscopy is carried out to determine injection barriers at the interface. The results from these two techniques are compared and the implications of which will be discussed.

Research paper thumbnail of Spectroscopy and imaging of metal–organic interfaces using BEEM

Applied Surface Science, 2006

Charge injection from metal electrodes to organics is a subject of intense scientific investigati... more Charge injection from metal electrodes to organics is a subject of intense scientific investigation for organic electronics. Ballistic electron emission microscopy (BEEM) enables spectroscopy and imaging of buried interfaces with nanometer resolution. Spatial non-uniformity of carrier injection is observed for both Ag-PPP (poly-paraphenylene) and Ag-MEHPPV (poly-2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene) interfaces. BEEM current images are found to correlate only marginally with the surface topography of the Ag film. #

Research paper thumbnail of Switching in organic devices caused by nanoscale Schottky barrier patches

Chemical Physics, 2005

We have identified a possible electronic origin of metal filaments, invoked to explain the switch... more We have identified a possible electronic origin of metal filaments, invoked to explain the switching behavior of organic devices. Interfaces of two representative organics polyparaphenylene ͑PPP͒ and poly͑2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene͒ with Ag are investigated using ballistic emission microscopy. Nanometer scale spatial nonuniformity of carrier injection is observed in ballistic electron emission microscopy images of both interfaces. The measured Schottky barrier ͑SB͒ appears to be consistent with metal states tailing into the gap of the PPP. We find that the SB values exhibit a distribution, even for the diodes with low ideality factors. The implications of this distribution on the measured physical properties of the diode are discussed, in light of work on devices of similar geometry, published in the literature. We also demonstrate that patches of low SB are likely to nucleate current filaments which can cause local ionization and are reported to be responsible for the switching behavior observed in metal-organic, metal-CuS and Ag-AgSe structures.

Research paper thumbnail of Ballistic electron microscopy of a metal molecule interface

We present studies on a Ag/ HS–(CH2)4–T3–H (T3C4SH)/ Au diode using nanometer scale resolution, b... more We present studies on a Ag/ HS–(CH2)4–T3–H (T3C4SH)/ Au diode using nanometer scale resolution, ballistic electron emission microscopy (BEEM). Images show spatially non-uniform carrier injection. A WKB calculation is carried out and compared with the experimental data. The results indicate that molecular levels are being accessed in the BEEM experiment, since the measured currents are larger than purely tunneling contribution. Our results are consistent with previously published results on a similar molecule [ 1]. Physical origins of the non-uniform carrier injection and its implications are discussed.

Research paper thumbnail of Ballistic emission spectroscopy and imaging of a buried metal/organic interface

Applied Physics Letters, 2005

The silver-p-phenylene (Ag-PPP) interface is investigated using ballistic electron emission micro... more The silver-p-phenylene (Ag-PPP) interface is investigated using ballistic electron emission microscopy (BEEM). Multiple injection barriers and spatial nonuniformity of carrier injection over nanometer length scales are observed. No unique injection barrier is found. Physical reasons for these features are discussed. BEEM current images and the surface topography of the silver film are uncorrelated.

Research paper thumbnail of Possible transition from space-charge-limited to injection-limited conduction in poly(3-hexylthiophene) thin films

Applied Surface Science, 2006

Two-terminal thin films of poly(3-hexylthiophene) (P3HT) with a wide electrode separation (150 mm... more Two-terminal thin films of poly(3-hexylthiophene) (P3HT) with a wide electrode separation (150 mm) has been studied using current-voltage characteristics at different temperatures. Space-charge-limited conduction (SCLC) with high injection barriers (1.3 eV) has been observed at all temperatures in the low electric field regime. A possible transition from SCLC to injection-limited conduction (ILC) is reported. The experimental results have been compared with the disorder-controlled injection model proposed by Arkhipov et al.

Research paper thumbnail of Erratum: Switching in organic devices caused by nanoscale Schottky barrier patches