Principle Scientist: Dr. Yongan Yang
Y. Yang, S.C. Kung, D.K. Taggart, C. Xiang, F. Yang, M.A. Brown, A.G. Güell, T.J. Kruse, J.C. Hemminger, R.M. Penner*, Synthesis of PbTe Nanowire Arrays using Lithographically Patterned Nanowire Electrodeposition, Nano Letters (2008) 2447.pdf.
In this Letter we describe the electrochemical synthesis of horizontal PbTe nanowire arrays that are electrically addressable, programmable in terms of their 2-dimensional contour on the surface, their height and width, and suspendable, enabling 25 micron sections of these nanowires to be suspended by 1-2 microns from the underlying surface, Fig 1. We use a method called lithographically patterned nanowire electrodeposition (LPNE) to achieve these results. Previously, LPNE has been used to pattern nanowires of noble metals (Au, Pd, and Pt) on glass. In LPNE, photolithography is used to prepare a 3-sided nano-form into which a nanowire can be electroplated using a horizontal nickel edge located within this nano-form.
The new method described here, called photoresist-bottomed LPNE or PB-LPNE, differs from LPNE in three ways: 1) A photoresist (PR)-covered glass surface serves as the starting point for this process. The thickness of this initial PR layer, typically 1.0 to 2.0 micron, determines the suspension height of the nanowires at the end of the PB-LPNE process, 2) A cyclic electrodeposition-stripping protocol is used to synthesize highly crystalline, stoichiometric PbTe nanowires within the nano-forms, and, 3) After the nanowires are synthesized and the outermost PR layer is removed, the PR supporting layer is photopatterned and developed to produce suspended sections of PbTe nanowires 25 microns in length. Importantly, PB-LPNE requires no dry etching that is damaging to PbTe. LPNE is also tolerant of air-borne contamination: All of the results presented here were achieved in a standard chemistry laboratory that was not equipped with specialized air filtration.
Figure 1. Synthesis of air-suspended PbTe nanowires. a,b) SEM images of suspended PbTe nanowire arrays at 2 micron pitch density. The air gap between the PbTe nanowires and the glass surface is 2 micronsin this case.
Figure 2. Schematic depiction of the twelve step process flow employed in the photoresist bottomed LPNE method. This new method, called photoresist-bottomed LPNE or PB-LPNE, permits air-suspended nanowires of PbTe to be fabricated. The PB-LPNE synthesis of air-suspended PbTe nanowire arrays was carried out according to the twelve-step process flow shown in Fig S1: Step 1) a 1-2 micron thick photoresist (PR) layer (Shipley S1800 series) was spin-coated onto a glass or oxidized silicon surface, Step 2) A nickel layer 20 - 100 nm in thickness was thermally deposited, Step 3) a positive-tone PR was coated onto this nickel layer and, Step 4) photopatterned using 365 nm illumination and a contact mask, Step 5) the exposed PR was removed, Step 6) now the remaining PR on the same was flood-exposed. A crucial point is that after this flood-exposure, no developing is carried out, Step 7) the exposed nickel was etched using HNO3 to produce a 500 nm deep undercut at the edges of the exposed regions (h). The horizontal trench formed by this undercut is the "nano-form" into which the PbTe nanowires will be electrodeposited. The height of this nano-form equaled the thickness of the nickel layer which defines the vertical back of the trench. This nano-form follows the contour of the photopatterned region. Step 8) the PbTe nanowire is electrodeposited into the trench (see procedure next paragraph), Step 9) the previously exposed top-PR layer was removed using developer, Step 10) the nickel layer was removed by etching in HNO3 leaving free-standing PbTe nanowires on top of an intact and unexposed layer of bottom-PR, Step 11) this bottom PR layer was exposed as before, Step 12) the exposed PR was removed leaving suspended PbTe nanowire segments up to 25 m in length. The combination of step 6 (exposure) and step 9 (developing), separated across the nanowire-electrodeposition process of steps 7 and 8 is the key for obtaining suspended nanowires in step 12. The "pre-exposure" of the top PR layer in step 6, at a point where the bottom PR layer is fully shielded from UV illumination, permits the selective removal of the top PR layer without disturbing the bottom PR layer. Omission of steps 11 and 12 produces PbTe nanowires on a planar photoresist surface.
Figure 3. Characterization of PbTe nanowires made via LPNE. A: A TEM image of an array of PbTe nanowires. Red arrows indicate the growth direction for one pair of wires; B: SAED (selected energy electron diffraction) pattern for the wires shown in (A) with assignments of six reflections. C: XRD (x-ray diffraction) patterns of PbTe nanowire arrays (a). A large bulge at low angles is contributed by the glass substrate. A PbTe film prepared using the same protocol as employed for nanowire growth, (b) indexed to fcc-cubic PbTe (JCPDS 38-1435); D: XPS (X-ray photoelectron spectroscopy) spectrum of Pb (4f) and Te (3d) taken from a PbTe nanowire array. Black: experimental data; Green: component 1; Blue; component 2; and Red: sum of green and blue curves.
In summary, stoichiometric and single phase PbTe nanowire arrays have been prepared using a new method, PB-LPNE, that provides for control over wire width and thickness, photolithographic patterning, and the suspension of nanowires across 25 m air gaps separating photoresist supports. The resulting nanowire-substrate system provides new opportunitities for the investigation of optical, electrical, and thermoelectric properties. Read more about the spectroscopy and conclusions in our manuscript.
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