NANO LETTERS
Asymmetric Photoconductivity within Nanoscale Break Junctions
2003 Vol. 3, No. 11 1561-1564
Tae-Hee Lee,† Chad R. Hladik,† and Robert M. Dickson*,†,‡ School of Chemistry and Biochemistry and Center for AdVanced Research in Optical Microscopy, Georgia Institute of Technology, 770 State Street, Atlanta, Georgia 30332-0400 Received September 3, 2003; Revised Manuscript Received October 3, 2003
ABSTRACT Electrically written silver oxide nanoscale break junctions clearly show asymmetric, wavelength-dependent photoconductivity, but only when either the anode oxide or the silver nanoclusters spanning the junction are illuminated. The higher oxygen content in the cathode increases the cathode oxide band gap and inhibits production of photoinjecting Ag nanoclusters. The optical and electronic properties of silver and its oxides suggest that useful nanoscale optoelectronic components can be created through a very simple one-step electromigration process.
Electromigration processes comprise a major decay route for electronic devices.1-3 This failure of metallic electrodes occurs through void formation and eventual expansion to a breakage. Recently, electromigration has been employed to fabricate nanoscale gaps for molecular electronics and optoelectronic applications.4-6 Such controlled electrode breakage yields relatively reproducible nanoscale junctions consisting of two metallic electrodes separated by several nanometers.4,6,7 Although seemingly symmetric, molecular electronic devices based on electromigration-induced junctions typically show asymmetric I-V characteristics.6-9 The origin of this asymmetry still remains an open problem. Here we report the inherent heterojunction nature of electromigration-induced silver oxide nanogap junctions as revealed through nanoscale junction photoconductivity. While having important implications for nanoscale and molecular electronics/optoelectronics,6,7,10,11 this asymmetric photoconductivity can be utilized on the nanoscale to detect photons and their wavelengths simply by using the ratio between the photoconductivities with forward and reverse bias. Silver oxide thin films for electromigration-induced nanogap junction fabrication were prepared by either chemical bath deposition (∼80 nm thickness)12 or RF sputtering (15∼50 nm thickness)13 on thin glass coverslips. All experiments were performed under moderate vacuum (10-5 Torr). After a given film is processed to a thin strip (100 µm width), direct current is applied through the strip by applying typically 1.0∼3.0 V in the case of chemical films, or 1.0∼40 V in the case of RF-sputtered films. The chemical bath deposition method yields 102∼103 Ω initial film resistance * Corresponding author. E-mail:
[email protected]. † School of Chemistry and Biochemistry. ‡ Center for Advanced Research in Optical Microscopy. 10.1021/nl034732b CCC: $25.00 Published on Web 10/18/2003
© 2003 American Chemical Society
measured across a 1-mm distance. After electromigration, resistance increases to typically 106 Ω with the chemically deposited films. The oxygen content of RF-sputtered films can be finely controlled to give different initial film resistances (104∼106 Ω). The RF sputtered films used here were sputtered with 200∼300 W RF power in a 1:2.0∼1: 3.0 Ar/O2 gas environment. These films consistently break with 15∼20 V electromigration voltage to give 107∼109 Ω final resistance. For all films, junction formation simultaneously produces silver nanoclusters in situ within the junction. Although not shown herein, formed nanoclusters emit strong electroluminescence (EL) under DC, AC, and pulsed excitation.5,8,9 Especially with pulsed excitation, the electroluminescence from the silver nanoclusters shows strong asymmetry with applied electric field polarity. The polarity dependence of the EL is partly from the asymmetric arrangement of formed nanoclusters during electromigration caused by the electron “wind”.3 This geometric proximity of the nanoclusters to the anode during the electromigration process yields the asymmetry in the EL.8,9 Here, we demonstrate that photoconductivity (the inverse of EL) more directly reveals the asymmetry inherent in the nanometerspaced electrodes created through electromigration. Under filtered mercury lamp excitation, only the anode silver oxide region is strongly and stably fluorescent, indicating that the two sides of the formed junction are now composed of different materials. The anode is defined as the electrode holding the higher potential during the inherently asymmetric electromigration process (Figure 1). Electron energy dispersive spectroscopy (EDS) shows that the anode side of the junction has lower oxygen content than does the cathode side (Figure 1C), indicating that the nanogap junction is naturally a heterojunction caused by the DC
Figure 1. Microscopic images and elemental composition of a nanogap heterojunction made from a silver oxide thin film. CCD images were taken with a 100× 1.4 NA oil immersion objective and Hamamatsu Orca 3 chip color CCD camera. (A) Transmitted light CCD image of the junction. No obvious junction is visible. (B) Combined fluorescent and transmitted light CCD image of the junction using room light and band-pass filtered mercury lamp excitation (450∼480 nm) through a 515 nm long pass emission filter. Only the anode side of the junction is strongly fluorescent. The strongly fluorescent nanoclusters within the junction also show EL under an external electric field.9 (C) Corresponding elemental composition of the junction taken with a Noran EDS system attached to a Hitachi 3500H SEM. The anode has lower oxygen content than does the cathode, thereby yielding the polarity of this junction from the energy level asymmetry.
electromigration process. As confirmed with RF-sputtered films of varying oxygen content, higher oxygen content in silver oxide generally leads to lower conductivity and slower conductance increase with temperature. Therefore, the cathode oxide barrier in the junction must have a wider band gap due to its higher oxygen content. Since both the anode and cathode oxides are composed of the same elements, the wider cathode oxide band gap indicates that it has a higher conduction band edge compared to that of the anode oxide barrier. This energy level asymmetry of the anode and cathode oxide barriers should also yield asymmetric charge transport through the junction. The high power necessary to produce DC-excited EL precludes the observation of polarity-dependent EL, unless low power pulsed or AC excitation is used.9 Photoconductivity,14,15 however, is readily observable as junction illumination produces a notable photocurrent under an applied external field (Figure 2). The photocurrent from the nanogap 1562
Figure 2. Photocurrent from a nanoscale silver oxide heterojunction with silver nanocluster tunneling bridges. An RF sputtered silver oxide film (200W RF power, deposition for 420 s in 1:2 Ar/O2 environment) is applied with 15 V electromigration voltage. Initial resistance and current are tunable with creation conditions. Photocurrent of the junction was measured under chopped (10 Hz) 514 nm excitation (125 W/cm2) with 12 V external field applied. (A) Photoconductivity response of the junction. The current reproducibly changes from 113 nA to 121 nA when the junction is optically excited by the chopped laser. The on and off photocurrent levels (the green line in the chart) are in good agreement with those at much slower rates with a slow mechanical shutter (∼1 Hz) blocking the excitation source. No hysteresis from thermal effects is detected up to 3000 seconds of continuous excitation (the photoinduced current remains same), although the baseline current increases over time sometimes up to 40 times higher than the initial value (not shown) due to the photoactivation of silver oxide to better conducting silver nanoclusters.16 (B) Photoconductivity response time of the junction. A mechanical chopper was used to measure the response time of the junction photoconductivity. Approximately 1000 measurements during 18 s illumination periods were made to give one point in the chart. Values of the current (baseline current, photocurrent, and the average photocurrent) are in good agreement with the cases shown in A. Limited by the chopper frequency and macroscopic laser beam size (3 mm diameter), the photoinduced current response was in all cases faster than we could measure. Nano Lett., Vol. 3, No. 11, 2003
junction is observed only when the anode silver oxide area or the nanoclusters within the gap are illuminated. As Ag nanoclusters are the photoinjecting species, the lack of nanocluster fluorescence on the cathode oxide underscores the fact that no photocurrent is observed by illuminating the cathode oxide area. This also corroborates the material asymmetry of the junction. The photoconductivity response time of the nanogap junction is very fast and shorter than 263 µs - the fastest chopper speed used (Figure 2B). Additionally, 3000 seconds of continuous excitation did not cause any thermal hysteresis, although the absolute value of the conductivity increases as the illuminated silver oxide region is photoreduced to a better conducting silver nanocluster region (not shown).16 As indicated from the lack of hysteresis with 3000 second continuous excitation, the photocurrent results from photoinduced charge carriers in the fluorescent anode oxide region, not thermal effects. Photoexcited silver nanoclusters within the junction can also contribute to the current. After being excited, the electrons flow toward one direction and holes toward the opposite direction to form a closed circuit. Since the junction is asymmetric, the photocurrent with forward and reverse bias should be different. To examine this photocurrent asymmetry, Hg-arc lamp excitation is used with both forward and reverse bias (Figure 3). With forward bias applied, excited electrons in the anode oxide region and within the junction will be excited and flow to the silver electrode. Thus, the amount of photocurrent with different photoexcitation wavelengths should be proportional both to the excitation profile of Ag nanoclusters17 and to the density of states (DOS) in the conduction band of the anode oxide to which the electrons are injected. Since the silver nanocluster excitation profile shows the highest efficiency under blue excitation,17,18 blue excitation is expected to yield the highest photocurrent efficiency with forward bias. With reverse bias applied, however, the excited electrons must tunnel to the cathode across the cathode oxide with its higher conduction band edge than that of the anode oxide. Consequently, in the reverse bias case, the photocurrent is proportional to the energy of the incoming photons (i.e., to that of excited electrons). As seen in Figure 3, the strong asymmetry in photoconductivity demonstrates that the higher conduction band of the cathode to which the excited electrons must tunnel is accessible only when the optically excited electron has high enough energy. As a result, the ratio of reverse to forward photoconductivity (i.e., the symmetry of the photoconductivity) depends on the energy of the photons exciting the nanoclusters (Figure 3). Thus, this asymmetry in the photoconductivity can be utilized to measure the wavelength of photons at the nanoscale. We clearly demonstrate the origin of asymmetric charge transport through a electromigration induced nanogap junction as well as the asymmetric photoconductivity without putting additional molecules in the gap. Simply by applying DC current on a single component silver oxide film, one can easily create optically responsive nanogap heterojunctions with potential optoelectronic applications such as photon detectors and an approximate wavelength meter. Nano Lett., Vol. 3, No. 11, 2003
Figure 3. Photoconductivity asymmetry of the silver oxide junction with varying excitation energy. Initial experimental conditions are as in Figure 2, except the excitation wavelengths and intensities. Using a 100-W Hg lamp in an epi-illumination geometry, 510∼550 nm, 450∼480 nm, and 360∼390 nm band-passes were used for green, blue, and UV excitation, respectively. Reflecting the heterojunction behavior, the symmetry of photoconductivity (ratio of photoconductivities with reverse and forward bias) increases as photon energy increases - 0.15 for green, 0.41 for blue, and 0.63 for UV. The inset shows the schematic energy diagram of the junction showing asymmetric band edge levels.19-21 The difference in silver oxide conduction band edges yields the photoconductivity asymmetry under forward and reverse bias. The higher cathode oxide conduction band edge provides a threshold that preferentially enables only higher energy photoelectrons to produce current in reverse bias. An Ag3 nanocluster is used as an example of a tunneling bridge.8
These results will also likely contribute to the improved measurement of charge transport dynamics within similarly fabricated molecular devices. Acknowledgment. The authors thank Olympus America Inc. for equipment loan, and funding from NSF (CHE9984507), the Dreyfus, and Sloan Foundations, and the Blanchard Endowment for junior faculty. References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
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NL034732B
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