Anthraquinone-2-carboxylic Acid Deposited from Solution onto Native Oxide Surfaces of Al Films Characterized by Inelastic Electron Tunneling Spectroscopy, X-ray Photoelectron Spectroscopy, and Atomic Force Microscopy

Abstract

Anthraquinone-2-carboxylic acid (AQ-2-COOH) deposited from acetone solutions onto native oxide surfaces of Al films was characterized by inelastic electron tunneling spectroscopy (IETS), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). These surface analyses show that AQ-2-COOH is adsorbed on the oxide as both a uniform film of anions on a nanometer scale and micron-sized particles of neutral molecules.IETS is a unique analytical technique of a thin (nm) insulator film of a metal/insulator/metal (MIM) tunneling junction and reveals the vibrational spectrum of the insulator at cryogenic temperatures.1-3 The high sensitivity and resolution as well as the wide spectral range enable us to obtain a detailed vibrational spectrum of adsorbed species on the oxide surface of the metal electrode. Aluminum is widely used as the metal electrode because of chemical, physical, and electric stability of the oxide. 1-3Thus, IETS is a powerful surface analytical technique and has been used in many fields of chemistry. [1][2][3] In the present letter, we report the tunneling spectra of AQ-2-COOH on the oxide surfaces of Al films for the first time. The tunneling spectra were compared with the vibrational spectra of AQ-2-COOH on Ag 4,5 and Au 6 by reflection absorption infrared spectroscopy (RAIR). The adsorption state and the morphology of AQ-2-COOH on the oxide surfaces of Al films were also investigated by XPS and AFM, respectively. Aluminum (99.999%) was evaporated on a glass slide (13 Â 37 Â 1 mm 3 ) at room temperature to form strips (1 mm wide) in a high vacuum (10 À3 Pa) and the surfaces were oxidized to form the oxide (alumina) in an oxygen dc glow discharge. One drop of acetone solutions (0.01-0.02 mg mL À1 ) of AQ-2-COOH (Tokyo Kasei, >99:0%) was dropped on the strips and excess solution was spun off. The junctions were completed with an evaporated Pb cross strip (1 mm wide). Tunneling spectra were measured at the liquid-helium temperature (4.2 K) with the tunneling spectrometer already reported.2,3 Aluminum was evaporated on mica sheets (13 Â 13 Â 0:1 mm 3 ) and the surfaces were also oxidized. AQ-2-COOH was deposited on the alumina surfaces from acetone solutions of 0.01-0.10 mg mL À1 at room temperature by the spin doping. 7,8 The deposited AQ-2-COOH was investigated by XPS (Shimadzu ESCA-1000, Mg K radiation) 8 and AFM (Digital Instruments NanoScope III, contact mode). 7,8The tunneling spectrum of AQ-2-COOH doped from an acetone solution (0.02 mg mL À1 ) and that of the alumina doped from acetone are shown in Figure 1. The spectrum of the acetone-doped alumina is identical to that of undoped alumina, [1][2][3] showing that acetone is an appropriate solvent for measuring the tunneling spectra of AQ-2-COOH on alumina. Though the tunneling spectrum of AQ-2-COOH shows the peak of the quinone carbonyl groups (C=O) at 1630 cm À1 , it shows no peaks of the carboxylic acid group. On the other hand, it has the peak of the symmetrical stretching mode of the carboxylate group ( s COO À ) at 1390 cm À1...

Adsorption State and Morphology of Anthraquinone-2-carboxylic Acid Deposited from Solution onto the Atomically-Smooth Native Oxide Surface of Al(111) Films Studied by Infrared Reflection Absorption Spectroscopy, X-ray Photoelectron Spectroscopy, and Atomic Force Microscopy

“…9 The tunneling spectra of AQ-2-COOH doped from the same solution showed no peaks attributed to the carboxylic acid (COOH) group, however, they did have peaks attributed to the νsCOO -at 1390 cm -1 . 36 The tunneling spectra show that AQ-2-COOH is adsorbed on the alumina as carboxylate anions. The transmission IR spectra of both Na and Ag salts show strong peaks attributed to the asymmetrical stretching mode of the carboxylate group (νasCOO -) at 1605 -1606 cm -1 , however, the IRAS spectrum obtained from the solution (0.02 mg ml -1 ) have no corresponding peak because of the selection rule on the metal surface.…”

“…The peak in the histogram represents the distribution of zij and its width (full width at half maximum, W) is a good measure of the surface roughness. [30][31][32][33][34][35][36] If zij distributes randomly around the surface height at 0 nm, the resulting peak has a Gaussian shape with the standard deviation Rms. Thus, W is represented by 2.36Rms and most (>95%) areas of the peak are in the 2W region between ±W.…”

“…The adsorption state is consistent with that obtained from the tunneling spectra. 36 Both the tunneling spectra and IRAS spectra of AQ-2-COOH on oxidized Al films show that the carboxylic acid group reacts with the OH group on the alumina surface as an acid-base reaction to give symmetrically adsorbed carboxylate anions on the Lewis-acid sites (Al + ) on the surface, as shown in Fig. 3.…”

“…36 In this case, the alumina was prepared on evaporated Al films at room temperature in an oxygen-dc glow discharge. The adsorption state and morphology of AQ-2-COOH on alumina surfaces have also been investigated by XPS and AFM.…”

“…The adsorption state and morphology of AQ-2-COOH on alumina surfaces have also been investigated by XPS and AFM. 36 AQ-2-COOH was adsorbed onto the alumina surfaces as a uniform film of carboxylate anions deposited from acetone solutions with concentrations of 0.01 -0.02 mg ml -1 . AQ-2-COOH was deposited as micron-sized particles of neutral molecules deposited from acetone solutions with concentrations higher than 0.02 mg ml -1 .…”