Monochromatic Electron Photoemission from Diamondoid Monolayers

Diamondoids are molecules with cage structures that can be superimposed upon bulk diamond. These diamond-crystal cages are fused together and terminated by hydrogen. Adamantane is the smallest member, consisting of a single diamond cage; tetramantane, a higher diamondoid, is constructed by four cages. Since the discovery of higher diamondoids in petroleum, together with selective functionalization techniques (1–3), these materials have attracted interest because of their potential to combine the properties of diamond and nanomaterials. In addition to the optical, mechanical, and thermal properties of bulk diamond, hydrogen-terminated diamond surfaces have negative electron affinity (NEA), an important consideration in the development of electron emitters [reviewed in (4)]. However, difficulties in emission uniformity, electron injection, and electron transport have hindered the use of bulk diamond for this purpose (4). The primary challenge for electron emitters therefore remains to find a material that would realize uniformly large, highly efficient, and highly monochromatic electron emission. Diamondoids are an interesting candidate for electron emission, because they are essentially fully hydrogen-terminated diamond clusters. Indeed, recent quantum Monte Carlo calculations predict NEA for diamondoids up to 1 nm in size (5).

Large-area self-assembled monolayers (SAMs) of a functionalized diamondoid, [121]tetramantane-6-thiol (2, 3), were assembled on Ag or Au substrates (6, 7). Thin films of unsubstituted [121]tetramantane were also fabricated by evaporating tetramantane powder at 50° to 80°C onto cleaned Au substrates in situ. Photoemission spectroscopy (PES) and near-edge x-ray absorption fine structure (NEXAFS) measurements were performed (6) to analyze electron-emission properties and the molecular orientation of the SAMs. Several different samples with different bias voltages (0 to 9 V) were measured to check reproducibility. We observed obvious changes in the spectra after certain periods of beam exposure. All PES data shown here were collected at 30 K with less than 30 min of x-ray exposure.

The molecular orientation of the [121] tetramantane-6-thiol SAMs was characterized by polarization-dependent NEXAFS, a technique used extensively to study alkanethiolate SAMs (8–10). Figure 1A shows the total electron yield NEXAFS spectra of [121]tetramantane-6-thiolate on Au. The NEXAFS spectra resemble those from bulk and gas-phase [121]tetramantane (11, 12), indicating a high-purity film of tetramantanethiol adsorbed on the surface. The angular dependence seen in the NEXAFS spectra implies that the tetramantanethiol forms well-ordered monolayers with a preferential orientation. Absorption intensity of a NEXAFS resonance from a C 1s orbital depends directly on alignment between the linearly polarized incident x-rays and the transition dipole moment into a particular unoccupied orbital (8, 9). The angular dependence is simulated by summing over each atomic center with transition dipole moments oriented along the bonds (8, 9). These simulations agree with the experimental data (Fig. 1B) when the tetramantanethiol, with the z axis of the molecule defined by the S-C bond, is tilted by 30 ± 10° from the surface normal (Fig. 1C) (6). Additionally, the affinity of the thiol for the metal leads to thiolate-bound monolayers (7, 13–15), as confirmed by S 2p core-level x-ray photoelectron spectroscopy (16) (fig. S2).

Fig. 1. (A) Polarization-dependent total electron yield NEXAFS spectra collected on [121]tetramantane-6-thiol SAMs prepared on Au. Total electron yield (TEY) is plotted against photon energy at different beam incident angles. The leading-edge peak at 287.6 eV (red oval) is assigned to transitions from the C 1s core level to the unoccupied (C-H) * orbitals, and the peak at about 292.5 eV is assigned to (C-C) * orbitals (8, 10, 11). The second gap indicated by the arrow is the characteristic signature of diamondoids (11, 12). (B) Comparison between experiments and theoretical simulations. Red squares represent the experimental ratio of (C-H) * spectral weight between data at different angles and that at 20°. The black line is the calculated ratio based on the molecular geometry as shown in (C), with a polar angle of 36.5° (6). [View Larger Version of this Image (43K GIF file)]

The key results of this work are seen in the PES spectra of [121]tetramantane-6-thiol SAMs grown on Ag (Fig. 2A) and Au (Fig. 3A) substrates. An emission peak appears for both surfaces at about 1-eV kinetic energy, the onset of the spectra at low kinetic energy. The intensity of the peak exceeds all the valence band features. For SAMs grown on Ag and Au, the sharp peak comprises about 68% and 17% of the total electron yield, respectively. This peak intensity is several times as strong as that found for hydrogen-terminated diamond surfaces [e.g., (4, 17–19)]. Even with a logarithm plot (insets), one can still see a sharp feature instead of the typical exponential decay of secondary electrons in this energy range.

Fig. 2. (A) Photoelectron spectra of [121]tetramantane-6-thiol SAMs grown on Ag substrates, collected with 55-eV photon energy. The strong peak at 1-eV kinetic energy contains 68% of the total photoelectrons. The dotted line is a 50-times enlargement of valence band features. The inset shows the same spectra in a double logarithm plot. (B) Photoelectron spectrum collected on [121]tetramantane-6-thiol SAM covered by C60 sublimed in situ. The strong electron-emission peak disappears after C60 coverage. (C) Photoelectron spectrum collected on an annealed [121]tetramantane-6-thiol SAM. As in (B), the peak observed for the pristine SAM vanishes after the in situ annealing to 550°C. The difference between the spectrum in (C) and a pure Ag PES spectrum could be partially due to residual S atoms still bound to the surface after annealing. [View Larger Version of this Image (23K GIF file)]

Fig. 3. (A) Photoelectron spectra of [121]tetramantane-6-thiol SAMs grown on Au substrates. The sharp peak at 1 eV contains about 17% of the total photoelectrons. The inset shows a double logarithm plot. (B) Photoelectron spectra of unsubstituted [121]tetramantane films prepared in situ on Au substrates. The inset is an enlargement of the low–kinetic energy part of the spectrum showing only a small peak. [View Larger Version of this Image (25K GIF file)]

To ensure that this unusual electron emission originates from the diamondoid monolayers, we applied two different techniques to cover or remove the monolayer in situ. Coating the diamondoid SAM with one monolayer of C₆₀, which was evaporated onto the SAM surface, caused the sharp emission feature to vanish (Fig. 2B). The valence band of the C₆₀ covered surface is neither from tetramantanethiol nor from C₆₀ (20), but the origin of these features is not clear at present. The diamondoid SAM was also removed by annealing a SAM sample to 550°C in situ. Given that the thermal stability of conventional alkane thiol SAMs is 70°C (21), this treatment should remove the diamondoid, and as shown in Fig. 2C, the low–kinetic energy peak completely disappears after annealing (22).

To investigate the importance of a monolayer of functionalized diamondoid versus a thin film of diamondoid, we compared [121]tetramantane-6-thiol SAMs with [121]tetramantane films. Figure 3B shows the PES spectrum of the [121]tetramantane film. The spectrum shows a small peak at low kinetic energy, in sharp contrast with the data for [121]tetramantane-6-thiol SAMs (Figs. 2A and 3A). Two factors may contribute to this difference. One is the poor electron conductivity within the thicker films versus that through the monolayers, and the other is the role that the thiol groups play in the SAM samples. Notably, this result indicates that the strong electron emission does not occur solely from the diamondoid surface, but that the metal substrate is also intimately involved in the process.

We further confirmed that the sharp peak remains at the same energy position with varying photon excitation energy (fig. S3). This rules out the possibility of core-level excitations and suggests that this sharp feature is not from electrons directly excited by photons, but from electrons accumulated at an intrinsic energy level of the molecules.

PES has been used widely for studying NEA materials, and a sharp feature at a low–kinetic energy threshold is often evident in the spectra of NEA (4, 17–19, 23, 24). Thus, the emission peak presented in this work provides direct evidence that certain functionalized diamondoids are NEA materials, consistent with the recent diffusion Monte Carlo (DMC) calculations (5). Moreover, the calculated DMC band gap, about 7 eV for tetramantane (5), is also consistent with the band gap estimated from our photoemission spectra, 6 to 8 eV (25). As further evidence of NEA, we evaporated slightly potassium (K) metal onto the SAM sample. In the PES spectrum of a K-covered [121]tetramantane-6-thiol SAM on a Ag substrate, the sharp peak retains its high intensity and occurs at the same energy position as in the PES spectrum without K (fig. S4). This is another indication of NEA because K deposition onto a positive electron affinity semiconductor will lead to a shift of the low–kinetic energy cutoff and strong enhancement of the secondary electron background.

On a typical NEA surface, electrons excited into unoccupied states relax to the bottom of the conduction band as a result of inelastic scattering, a process normally referred to as the secondary cascade. A number of secondary electrons will then accumulate at the bottom of the conduction band. For a surface with positive electron affinity (as occurs in almost all untreated semiconductor surfaces), these accumulated electrons cannot escape. For an NEA surface, these accumulated electrons can be emitted directly because the vacuum level lies below the bottom of conduction band. As a result, a peak will be observed at the low–kinetic energy threshold in PES (4, 17–19, 23, 24).

However, on diamondoid SAM surfaces, there is only a single layer of diamondoid molecules. The detailed mechanism responsible for the highly monochromatic emission is unknown at this stage. Naïvely, one may consider that photoexcited electrons lose energy by creating phonons in the molecules, but this would likely lead to the destruction of the molecules. A plausible scenario is that most of the photoexcited electrons come from the substrate. These electrons first thermalize in the metal, producing many more low-energy electrons. Electrons with energies above the diamondoid conduction-band minimum may get transferred to diamondoid molecules, reach the bottom of the conduction band by creating phonons, and get emitted. This proposal is shown schematically in Fig. 4. Another difference between our results and those of other typical NEA systems (4, 17–19, 23, 24) is that our data show a spike in the spectra rather an exponential rise of the secondary tail toward the threshold, suggesting that a single energy level, resulting from the molecular nature of nanometer-sized diamondoids, and/or a strong resonance process are involved.

Fig. 4. Schematic of the electron-emission process on diamondoid SAM surfaces. EF is the Fermi level of the metal substrate, sitting in the energy gap of diamondoid. The vacuum level (EVacuum) is below the conduction-band minimum of the diamondoid, a characteristic of NEA. The dotted red line depicts the high-probability electron emission. First, electrons in metal substrates are excited by photons into unoccupied states above EF. Second, the excited electrons effectively thermalize in the metal, producing more electrons with lower energy. Third, electrons with energy above the conduction band minimum are transferred to diamondoid moieties. These electrons further lower their energies by exciting phonons in the molecules, and they accumulate at the bottom of the conduction band. Finally, because of NEA, electrons accumulated at the bottom of the conduction band emit into vacuum spontaneously and generate a peak at the low–kinetic energy threshold. Electron emission takes place also at high–kinetic energy levels, but with much lower photoelectron yield. Although this scenario roughly explains the existence of the electron-emission peak, more theoretical understanding is needed to fully explain the results. [View Larger Version of this Image (32K GIF file)]

Our results suggest that diamondoid monolayers may have promising utility. Not only can functionalized diamondoids be easily grown into large area SAMs with NEA properties, they also naturally circumvent the long-standing electron-conductivity issues encountered for wide-gap bulk NEA semiconductors (4, 26). On a diamondoid SAM surface, electron conduction from the electron reservoir (metal substrates) to the emission surface is through a single molecule, which successfully avoids the low-conductivity problem and enhances the electron emission. Additionally, the possibility of different functionalizations (3, 4) allows one to optimize the NEA and other properties of diamondoids. Although many technical issues need to be addressed before diamondoid SAMs can be used as electron emitters, diamondoids provide intrinsic advantages over bulk materials because of their special molecular characteristics—for example, narrow energy distribution of the electronic states.

References and Notes

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