Phonon-Assisted Field Emission in Silicon Nanomembranes for Time- of-Flight Mass Spectrometry of Proteins Jonghoo Park, †,‡ Zlatan Aksamija, ‡ Hyun-Cheol Shin, § Hyunseok Kim, ‡ and Robert H. Blick* ,‡,§,∥ † Department of Electrical Engineering, Kyungpook National University, Daegu, Korea ‡ Department of Electrical and Computer Engineering and § Department of Material Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States ∥ Angewandte Physik, Universitat Hamburg, Jungiussstrasse 11, 20355 Hamburg, Germany * S Supporting Information ABSTRACT: Time-of-flight (TOF) mass spectrometry has been considered as the method of choice for mass analysis of large intact biomolecules, which are ionized in low charge states by matrix- assisted-laser-desorption/ionization (MALDI). However, it remains predominantly restricted to the mass analysis of biomolecules with a mass below about 50 000 Da. This limitation mainly stems from the fact that the sensitivity of the standard detectors decreases with increasing ion mass. We describe here a new principle for ion detection in TOF mass spectrometry, which is based upon suspended silicon nanomembranes. Impinging ion packets on one side of the suspended silicon nanomembrane generate nonequilibrium phonons, which propagate quasi-diffusively and deliver thermal energy to electrons within the silicon nanomembrane. This enhances electron emission from the nanomembrane surface with an electric field applied to it. The nonequilibrium phonon-assisted field emission in the suspended nanomembrane connected to an effective cooling of the nanomembrane via field emission allows mass analysis of megadalton ions with high mass resolution at room temperature. The high resolution of the detector will give better insight into high mass proteins and their functions. KEYWORDS: NEMS, nanomembrane, phonons, mass spectrometry I n time-of-flight (TOF) mass spectrometry, 1 the low charge state ions, such as those generated by the matrix-assisted laser desorption/ionization (MALDI) process, 2,3 are acceler- ated in an electric field and drift to a detector with different velocities. Although TOF mass spectrometry has been known to operate with an unlimited mass range, in practice, however, its mass range is limited by the sensitivity of the detector. The sensitivity of microchannel plate (MCP) detectors, which are used in most TOF mass spectrometers, decreases as v 4.4 , where v is the velocity of the incident ion. 4 This leads to a remarkable decrease in sensitivity of MCP detectors for heavier ions, which drift more slowly down to the detector than lighter ones. Phonon-mediated particle detectors such as cryogenic micro- calorimeters and superconducting tunnel junctions have been demonstrated to show mass (i.e., velocity) independent sensitivity by measuring the thermal energy deposited by ion bombardment at temperatures lower than hundred milli- kelvin. 5−12 Although these cryogenic particle detectors deliver exceptional mass sensitivity at high masses, the detectors require an expensive cryogenic cooling unit. Here we describe a nonconventional phonon-mediated particle detector for the detection of ultra large ions in TOF mass spectrometry operating at room temperature. Our approach is based upon nonequilibrium phonon-assisted field emission (PAFE) in silicon nanomembranes. The nano- membrane detector we describe here is illustrated in Figure 1a. the silicon nanomembrane is placed at the end of the flight tube of a commercial MALDI-TOF mass spectrometer (Perseptive Biosystems Voyager-DE STR). The detector consists of four parts, a silicon nanomembrane, an extraction gate, microchannel plates (MCPs), and an anode, as shown in Figure 1b. The silicon nanomembrane was fabricated from silicon-on-insulator (SOI) material by wafer thinning and wet etching to form an array of two suspended silicon nano- membranes with an area of (2 × 2) mm 2 each and a thickness of 180 nm. The operating principle of the detector is illustrated in a band diagram in Figure 1c. Applying an electric field at the surface of the silicon nanomembrane via the extraction gate lowers and “thins” the potential barrier, resulting in electron emission from the surface of the nanomembrane. The applied electric field we estimate to be about 1.9 × 10 7 V/m, which alone is not sufficient to place the nanomembrane in the strong Fowler− Nordheim tunneling regime. 13 Instead, the nanomembrane is in the so-called Schottky emission regime, 14,15 where both thermionic and tunneling components contribute, and the field emission current is well approximated by the expression 16 Received: March 8, 2013 Revised: April 17, 2013 Published: April 29, 2013 Letter pubs.acs.org/NanoLett © 2013 American Chemical Society 2698 dx.doi.org/10.1021/nl400873m | Nano Lett. 2013, 13, 2698−2703