Characterization of bio-hybrid interfaces under ambient conditions
The topography, morphology, and chemistry of the surface of HVPE-grown n-GaN(0001) layers on c-sapphire/n-GaN(0001) samples were explored under various conditions before and after sputtering with 1 keV nitrogen N2 ions, followed by the functionalization with deionized water (H2O) and neutral L-cysteine (C3H7NO2S) molecules. Atomic force microscopy (AFM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) in ultra-high vacuum (UHV) and near-ambient-pressure (NAP) conditions were used for this investigation. Monochromatic X-ray 𝐴𝐴𝐴𝐴 𝐾𝐾𝐾𝐾 radi ation (ℎ𝜋𝜋 = 1486.7𝑒𝑒𝑉𝑉) was the exc itat ion source for the XPS experiments. The results obtained yielded information on the modifications occurring at the surface of the n-GaN(0001) layers upon N2+-sputtering and on the contact formation and interfacial chemical reactivity after exposure to H2O and L-cysteine, i.e., on the ensuing interactions and bonding mechanisms between H2O and L-cysteine molecules and the N2+-sputtered n-GaN(0001) surface. This provided a clue on the suitability, stability, and functionality of GaN-OH and GaN-SH groups, the primary links between the surface and inorganic, organic and biomolecules, and hence, on the potentiality of using GaN-based surfaces for electronic devices and biosensors. Experimental UHV-XPS spectra of C1s and O1s core-level (CL) transitions of the as-grown n-GaN(0001) surface revealed adsorbed adventitious carbon (C) and oxygen (O2) species, besides traces of Ga2O3 and Ga2Ox oxides. The surface AFM images and LEED patterns revealed that N2+-ion sputtering resulted in minimal surface damage and roughness, in addition to largely reducing the amount of adsorbed carbon species. The NAP-XPS spectra of the Ga 2p3/2 and Ga 3d5/2 photoemission lines of the N2+-sputtered n-GaN(0001)/H2O interface showed that the H2O molecules adsorbed on the n-GaN(0001) surface likely dissociated into a hydroxyl ion (OH−) and a proton (H+), which tend to combine with surface gallium (Ga+) dangling bonds and unbonded nitrogen (N−) ions, respectively. The N2+-ion sputtering and H2O exposure modified the valence band (VB) structure of the as-grown n-GaN(0001) surface. This is shown by the respective UHV and NAP-XPS VB photoemission spectra, which displayed VB peaks related to the hybridized orbital states of apolar surface dominated by Ga, besides revealing a reduction in the band bending at the N_2^+-sputtered n-GaN(0001) surface. Functionalization of the N_2^+-sputtered n-GaN(0001) surface with H2O molecules induced a noticeable modification to its chemistry and band bending (BB), disclosed from the measured NAP-XPS O 1s and Ga 3d lines, and from the NAP-XPS VB spectra of the ensuing N_2^+-sputtered n-GaN(0001)/H2O interface. The formation of Ga-OH and N-H bonds, and also Ga-O bonds that become prominent at high temperatures, are inferred from the NAP-XPS peaks of the existing surface constituents. Such a compensation of dangling bonds and surface states upon increasing the amount of H2O exposure typified itself as a continuous decrease in the surface BB voltage V_bb up to a pressure p=0.1 mbar, above which V_bb leveled off at a constant value, depending on the sample temperature T. The surface BB voltage V_bb was estimated by two methodologies. One approach used the energy difference (E_FS-E_VS) between the surface valence-band edge (E_VS) and the Fermi level (E_FS), the VBM energy, by the linear extrapolation method. The other approach monitored the measured peak energy of the Ga 3dS line of the N_2^+-sputtered n-GaN(0001) surface and the N_2^+-sputtered n-GaN(0001)/H2O interface viz., (E_FS-E_Ga3dS), combined with 17.5 eV, the energy difference between VB edge and Ga 3dB CL of bulk GaN. The two approaches incorporated the energy differences between the conduction band (CB) edge (E_CB) and the Fermi energy E_FB of bulk GaN (E_CB-E_FB) and between E_CB and E_VB, the band gap energy E_g, both of which are temperature dependent. This partially explains the observed decrease in V_bb with increasing T. Due to the difficulty of finding a proper linear extrapolation for the VB spectra of the N_2^+-sputtered n-GaN(0001)/H2O interface, mainly at high H2O pressure, V_bb was estimated from the binding energy shifts of (E_FS-E_Ga3dS ), combined with (E_VB-E_Ga3dB )=17.5eV, and (E_FS-E_VS ), the VBM energy. At a constant T, the decrease in V_bb at the N_2^+-sputtered n-GaN(0001)/H2O interface with pressure p below 0.1 mbar is ascribed to the compensation of surface dangling bonds by dissociative H2O molecules. The constancy of V_bb for p>0.1 mbar is understood in terms of a saturation coverage of the n-GaN(0001) surface with H2O molecules. The reduction of p>0.1 V_bb with increasing T can be partly accounted for by the decrease in E_g (T)=(E_CB-E_VB ) and (E_CB-E_FB (T)) with T, though such a decrease cannot solely clarify the entire trend of V_bb at high T. The N_2^+-sputtered n-GaN(0001) surface permits notable bonding of the sulfur-containing amino-acid L-cysteine (L-CySH) molecules, as concluded from the respective UHV-XPS spectra. Deposition of L-CySH on the n-GaN(0001) surface was made via sublimation of L-CySH powder at 105 oC for ≤ 40 min. The UHV-XPS spectra of the N_2^+-sputtered GaN(0001)/L-cysteine interface were taken at room temperature and after annealing at various temperatures (T_A=75-150℃) for annealing times (t_A) of 15, 30, and 60 minutes. The UHV-XPS spectra of the N_2^+-sputtered n-GaN(0001)/L-cysteine interface showed that the thiol head group (SH group) of the L-cysteine molecule formed strong chemical bonding with the surface metallic Ga. Such behavior was noted from the overlapped Ga 3s and S 2p XPS photoemission lines, the intensity of which were enhanced with more deposited L-cysteine layers, a feature that is characteristic for thiolate species. The bonding of carboxyl (COOH) and amino (NH2) groups to the surface cannot be entirely excluded, as was noted from the respective C 1s, O 1s, and N 1s XPS spectra. The strength and appearance of the doublet S 2p photoemission lines diminished as T_A increased, suggesting that most but not all L-cysteine layers re-evaporated from the functionalized surface, with the SH-group of the lowermost L-cysteine layer being still chemically bonded to the Ga atoms at the surface.
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