Oleksandr Voznyy

Publications

• Theory of highly excited semiconductor nanostructures including Auger coupling: exciton-bi-exciton mixing in CdSe nanocrystals

  Marek Korkusinski, Oleksandr Voznyy, Pawel Hawrylak

  09/2011;

  We present a theory of highly excited interacting carriers confined in a semiconductor nanostructure, incorporating Auger coupling between excited states with different number of excitations. The Coulomb matrix elements connecting exciton, bi-exciton and tri-exciton complexes are derived and an intu... [more] We present a theory of highly excited interacting carriers confined in a semiconductor nanostructure, incorporating Auger coupling between excited states with different number of excitations. The Coulomb matrix elements connecting exciton, bi-exciton and tri-exciton complexes are derived and an intuitive picture of breaking neutral multi-exction complexes into positively and negatively charged multi-exciton complexes is given. The general approach is illustrated by analyzing the coupling of biexciton and exciton in CdSe spherical nanocrystals. The electron and hole states are computed using atomistic $sp^3d^5s^*$ tight binding Hamiltonian including an effective crystal field splitting and surface passivation. For each number of electron-hole pairs the many-body spectrum is computed in the configuration-interaction approach. The low-energy correlated biexciton levels are broken into charged complexes: a hole and a negatively charged trion and an electron and a positively charged trion. Out of a highly excited exciton spectrum a subspace coupled to bi-exciton levels via Auger processes is identified. The interaction between correlated bi-exciton and exciton states is treated using exact diagonalization techniques. This allows to extract the spectral function of the biexciton and relate its characteristic width and amplitude to the characteristic amplitude and timescale of the coherent time evolution of the coupled system. It is shown that this process can be described by the Fermi's Golden Rule only if a fast relaxation of the excitonic subsystem is accounted for.

• Mobile surface traps in CdSe nanocrystals with carboxylic acid ligands

  Oleksandr Voznyy

  J. Phys. Chem. C. 01/2011; 115:15927.

  We have performed ab initio calculations of electronic properties of the realistic Cd- rich CdSe nanocrystals with covalently bound carboxylic acid (X-type) ligands. Configurations both with and without surface traps can be prepared depending on the amount and geometry of the adsorbed ligands. We fi... [more] We have performed ab initio calculations of electronic properties of the realistic Cd- rich CdSe nanocrystals with covalently bound carboxylic acid (X-type) ligands. Configurations both with and without surface traps can be prepared depending on the amount and geometry of the adsorbed ligands. We find that Cd and Se dangling bonds do not necessarily create surface traps, while traps originating from ligands can form near the top of the valence band. Some of the ligands are found to be mobile on the surface and this mobility is accompanied by a spectral diffusion of the associated trap energy levels. This provides the first atomistic example of the processes required to explain the emission wavelength and lifetime variations, and blinking of the nanocrystals.

• Edge stability, reconstruction, zero-energy states and magnetism in triangular graphene quantum dots with zigzag edges

  Oleksandr Voznyy, Alev Devrim Güçlü, Pawel Potasz, Pawel Hawrylak

  11/2010;

  We present the results of ab-initio density functional theory based calculations of the stability and reconstruction of zigzag edges in triangular graphene quantum dots. We show that, while the reconstructed pentagon-heptagon zigzag edge structure is more stable in the absence of hydrogen, ideal zig... [more] We present the results of ab-initio density functional theory based calculations of the stability and reconstruction of zigzag edges in triangular graphene quantum dots. We show that, while the reconstructed pentagon-heptagon zigzag edge structure is more stable in the absence of hydrogen, ideal zigzag edges are energetically favored by hydrogen passivation. Zero-energy band exists in both structures when passivated by hydrogen, however in case of pentagon-heptagon zigzag, this band is found to have stronger dispersion, leading to the loss of net magnetization. Comment: 4.2 pages, 5 figures

• Fine structure and size dependence of exciton and bi-exciton optical spectra in CdSe nanocrystals

  Marek Korkusinski, Oleksandr Voznyy, Pawel Hawrylak

  09/2010;

  Theory of electronic and optical properties of exciton and bi-exciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic $sp^3d^5s^*$ tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactio... [more] Theory of electronic and optical properties of exciton and bi-exciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic $sp^3d^5s^*$ tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach. Results demonstrate that the low-energy electron spectrum resembles $s$ and $p$ shells as expected in a single-band effective mass approximation but the valence hole spectrum is composed of four low-lying, doubly degenerate states separated from the rest by a gap. As a result, the bi-exciton and exciton spectrum is composed of a manifold of closely lying states, resulting in a fine structure of exciton and bi-exciton spectra. The degenerate nature of the hole spectrum makes the bi-exciton energy and wave function sensitive to correlation effects. We find that the relative position of the exciton and bi-exciton peaks in the emission spectrum depends on the size of the basis, temperature, and diameter of the nanocrystal. Comment: 13 pages

• Gold adatom as a key structural component in self-assembled monolayers of organosulfur molecules on Au(111)

  Peter Maksymovych, Oleksandr Voznyy, Daniel B. Dougherty, Dan C. Sorescu, John T. Yates Jr.

  Progress in Surface Science. 05/2010; 85:206-240.

  Chemisorption of organosulfur molecules, such as alkanethiols, arenethiols and disulfide compounds on gold surfaces and their subsequent self-organization is the archetypal process for molecular self-assembly on surfaces. Owing to their ease of preparation and high versatility, alkanethiol self-asse... [more] Chemisorption of organosulfur molecules, such as alkanethiols, arenethiols and disulfide compounds on gold surfaces and their subsequent self-organization is the archetypal process for molecular self-assembly on surfaces. Owing to their ease of preparation and high versatility, alkanethiol self-assembled monolayers (SAMs) have been widely studied for potential applications including surface functionalization, molecular motors, molecular electronics, and immobilization of biological molecules. Despite fundamental advances, the dissociative chemistry of the sulfur headgroup on gold leading to the formation of the sulfur–gold anchor bond has remained controversial. This review summarizes the recent progress in the understanding of the geometrical and electronic structure of the anchor bond. Particular attention is drawn to the involvement of gold adatoms at all stages of alkanethiol self-assembly, including the dissociation of the disulfide (S–S) and hydrogen-sulfide (S–H) bonds and subsequent forma

• Molecular self-assembly and passivation of GaAs (001) with alkanethiol monolayers: A view towards bio-functionalization

  Jan J. Dubowski, Oleksandr Voznyy, G.M. Marshall

  Applied Surface Science. 04/2010; 256:5714-5721.

  Properties of as prepared or nanoengineered III–V semiconductor surfaces provide attractive means for photonic detection of different adsorbants from surrounding gaseous or liquid environments. To be prac- tical, this approach requires that the surface is made selectively sensitive (functionalized) ... [more] Properties of as prepared or nanoengineered III–V semiconductor surfaces provide attractive means for photonic detection of different adsorbants from surrounding gaseous or liquid environments. To be prac- tical, this approach requires that the surface is made selectively sensitive (functionalized) to targeted species. In addition, such surface has also to stay stable over extended period of time to make it available for rapid testing. Numerous reports demonstrate attractive properties of GaAs for sensing applications. One of the most fundamental issues relevant to these applications concerns the ability to functionalize chemically, or biologically, the surface of GaAs. The most studied method of GaAs surface functionaliza- tion is based on formation of self-assembled monolayers (SAMs) of various n-alkanethiols, HS-(CH2)n-T (T=CH3, COOH, NH2, etc.). In spite of multi-year research concerning this issue, it has only been recently that a comprehensive picture of SAMs formation on GaAs and an understand

• Fine structure and size dependence of exciton and biexciton optical spectra in CdSe nanocrystals

  Marek Korkusinski, Oleksandr Voznyy, Pawel Hawrylak

  Phys. Rev. B. 01/2010; 82:245304-245304.

  Theory of electronic and optical properties of exciton and biexciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic sp3d5s∗ tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, an... [more] Theory of electronic and optical properties of exciton and biexciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic sp3d5s∗ tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach. Results demonstrate that the low-energy electron spectrum is organized in shells (s,p,…), while the valence hole spectrum is composed of four low-lying, doubly degenerate states separated from the rest by a gap. As a result, the biexciton and exciton spectrum is composed of a manifold of closely lying states, resulting in a fine structure of exciton and biexciton spectra. The quasidegenerate nature of the hole spectrum results in a correlated biexciton state, which makes it slowly convergent with basis size. We carry out a
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  Magnetism and correlations in fractionally filled degenerate shells of graphene quantum dots.

  A D Güçlü, P Potasz, O Voznyy, M Korkusinski, P Hawrylak

  Physical review letters. 12/2009; 103(24):246805.

  We show that the ground state and magnetization of the macroscopically degenerate shell of electronic states in triangular gated graphene quantum dots depends on the filling fraction of the shell. The effect of degeneracy, finite size, and electron-electron interactions are treated nonperturbatively... [more] We show that the ground state and magnetization of the macroscopically degenerate shell of electronic states in triangular gated graphene quantum dots depends on the filling fraction of the shell. The effect of degeneracy, finite size, and electron-electron interactions are treated nonperturbatively using a combination of density functional theory, tight-binding, Hartree-Fock and configuration interaction methods. We show that electronic correlations play a crucial role in determining the nature of the ground state as a function of filling fraction of the degenerate shell at the Fermi level. We find that the half-filled charge neutral shell leads to full spin polarization but this magnetic moment can be completely destroyed by adding a single electron.
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  The role of gold adatoms and stereochemistry in self-assembly of methylthiolate on Au(111).

  Oleksandr Voznyy, Jan J Dubowski, J T Yates, Peter Maksymovych

  Journal of the American Chemical Society. 10/2009; 131(36):12989-93.

  On the basis of high resolution STM images and DFT modeling, we have resolved low- and high-coverage structures of methylthiolate (CH(3)S) self-assembled on the Au(111) surface. The key new finding is that the building block of all these structures has the same stoichiometry of two thiolate species ... [more] On the basis of high resolution STM images and DFT modeling, we have resolved low- and high-coverage structures of methylthiolate (CH(3)S) self-assembled on the Au(111) surface. The key new finding is that the building block of all these structures has the same stoichiometry of two thiolate species joined by a gold adatom. The self-arrangement of the methylthiolate-adatom complexes on the surface depends critically on their stereochemical properties. Variations of the latter can produce local ordering of adatom complexes with either (3 x 4) or (3 x 4 square root(3)) periodicity. A possible structural connection between the (3 x 4 square root(3)) structure and commonly observed (square root(3) x square root(3))R30 degrees phase in methylthiolate self-assembled monolayers is developed by taking into account the reduction in the long-range order and stereochemical isomerization at high coverage. We also suggest how the observed self-arrangements of methylthiolate may be related to the c(4 x 2) phase of its longer homologues.

• Magnetism and correlations in fractionally filled degenerate shells of graphene quantum dots

  A. D. Guclu, P. Potasz, O. Voznyy, M Korkusinski, P. Hawrylak

  07/2009;

  When an electron is confined to a triangular atomic thick layer of graphene [1-5] with zig-zag edges, its energy spectrum collapses to a shell of degenerate states at the Fermi level (Dirac point) [6-9]. The degeneracy is proportional to the edge size and can be made macroscopic. This opens up the p... [more] When an electron is confined to a triangular atomic thick layer of graphene [1-5] with zig-zag edges, its energy spectrum collapses to a shell of degenerate states at the Fermi level (Dirac point) [6-9]. The degeneracy is proportional to the edge size and can be made macroscopic. This opens up the possibility to design a strongly correlated electronic system as a function of fractional filling of the zero-energy shell, in analogy to the fractional quantum Hall effect in a quasi-two-dimensional electron gas[10], but without the need for a high magnetic field. In this work we show that electronic correlations, beyond the Hubbard model[6,7] and mean-field density functional theory (DFT) [7,8] play a crucial role in determining the nature of the ground state and the excitation spectrum of triangular graphene quantum dots as a function of dot size and filling fraction of the shell of zero-energy states. The interactions are treated by a combination of DFT, tight-binding, Hartree-Fock and configuration interaction methods (TB-HF-CI) and include all scattering and exchange terms within second nearest neighbors as well as interaction with metallic gate. We show that a half filled charge neutral shell leads to full spin polarization of the island but this magnetic moment is completely destroyed by the addition of a single electron, in analogy to the effect of skyrmions on the quantum Hall ferromagnet [11-14] and spin depolarization in electrostatically defined semiconductor quantum dots[15-18]. The depolarization of the ground state is predicted to result in blocking of current through a graphene quantum dot due to spin blockade (SB) [18]. Comment: v2: minor corrections, new format
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  c(4 x 2) Structures of Alkanethiol Monolayers on Au (111) Compatible with the Constraint of Dense Packing.

  Oleksandr Voznyy, Jan J Dubowski

  Langmuir : the ACS journal of surfaces and colloids. 06/2009;

  Using alkanethiol dense packing as a starting point, we have found six prototypical packing structures commensurate with the (3 x 2 radical3) supercell of the Au (111) surface. Five of the six structures are not compatible with the flat surface conditions but can be fitted to a reconstructed surface... [more] Using alkanethiol dense packing as a starting point, we have found six prototypical packing structures commensurate with the (3 x 2 radical3) supercell of the Au (111) surface. Five of the six structures are not compatible with the flat surface conditions but can be fitted to a reconstructed surface. Combined with density functional theory calculations and simulations of grazing incidence X-ray diffraction maps and of scanning tunneling microscopy images, this allowed us to refine and assess the recently proposed models of the c(4 x 2) self-assembled monolayers involving thiolate-adatom and thiolate-adatom-thiolate species and to propose a new model with four gold adatoms per unit cell.

• Magic-Sized Cd 3 P 2 II-V Nanoparticles Exhibiting Bandgap Photoemission

  Ruibing Wang, Christopher I Ratcliffe, Xiaohua Wu, Oleksandr Voznyy, Ye Tao, Kui Yu

  J. Phys. Chem. C. 01/2009; 113:17979-17982.

  The very first single-sized Cd3P2 II−V nanoparticles were synthesized via a non-injection-based approach which was designed to be thermodynamically driven. The Cd3P2 nanoparticles were synthesized in a pure form and exhibited bright bandgap photoemission peaking at 455 nm with a full width at half-m... [more] The very first single-sized Cd3P2 II−V nanoparticles were synthesized via a non-injection-based approach which was designed to be thermodynamically driven. The Cd3P2 nanoparticles were synthesized in a pure form and exhibited bright bandgap photoemission peaking at 455 nm with a full width at half-maximum (fwhm) of only 17 nm and narrow bandgap absorption peaking at 451 nm. Compared to those reported before with a fwhm of 50−150 nm, the newly developed Cd3P2 nanoparticles represent significant progress in synthesis with better design and control. Cadmium acetate dihydrate (Cd(OAc)2·2H2O) and tris(trimethylsilyl)phosphine ((TMS)3P) were used as Cd and P source compounds, respectively; the synthesis was carried out in 1-octadecene (ODE), a noncoordinating solvent. The novel Cd3P2 nanoparticles were further characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (PXRD), and 113Cd and 31P solid-state NMR. These single-sized Cd3P2 na
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  Structure of Thiol Self-Assembled Monolayers Commensurate with the GaAs (001) Surface.

  Oleksandr Voznyy, Jan Dubowski

  Langmuir : the ACS journal of surfaces and colloids. 11/2008;

  Observed properties of thiol self-assembled monolayers (SAMs) on GaAs (001) surfaces can be explained by the presence of surface reconstructions, but their exact form is generally unknown. We propose a new approach to modeling the SAM-surface interface based on using alkanethiol dense packing struct... [more] Observed properties of thiol self-assembled monolayers (SAMs) on GaAs (001) surfaces can be explained by the presence of surface reconstructions, but their exact form is generally unknown. We propose a new approach to modeling the SAM-surface interface based on using alkanethiol dense packing structures as a starting point and adjusting the surface reconstruction to accommodate them. Obtained in such a way, model SAMs adsorb along the trenches in the [110] direction and exhibit a 19 degrees tilt and +/- 45 degrees twist angles, in agreement with available experimental data. The molecules of the SAM bind to both Ga and As, and cover only 50% of the available surface sites. The requirements for the SAM formation process to achieve the proposed structures are discussed.

• Adsorption Kinetics of Hydrogen Sulfide and Thiols on GaAs (001) Surfaces in a Vacuum

  Oleksandr Voznyy, Jan J. Dubowski

  J. Phys. Chem. C. 03/2008; 112:3726-3733.

  Adsorption mechanisms of hydrogen sulfide and alkanethiols on GaAs (001) surfaces prepared in a vacuum were studied using first principles calculations. The need for a physisorbed precursor was confirmed based on energetic arguments and molecular dynamics simulations of the transition from a physiso... [more] Adsorption mechanisms of hydrogen sulfide and alkanethiols on GaAs (001) surfaces prepared in a vacuum were studied using first principles calculations. The need for a physisorbed precursor was confirmed based on energetic arguments and molecular dynamics simulations of the transition from a physisorption to a chemisorption state. The preference for S−Ga bond formation was found, resulting from a weak non-dissociative chemisorption state formed by the overlap of a sulfur lone pair orbital with an empty Ga dangling bond. Physisorption energies and the height of the transition barrier from physisorption to chemisorption on GaAs were found to be very similar to those on gold, while the main difference from gold was that hydrogen remains on the GaAs surface upon S−H cleavage. Obtained results allowed for a more advanced interpretation of experimental data concerning H2S and thiol adsorption on GaAs.
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  Structure, bonding nature, and binding energy of alkanethiolate on As-rich GaAs (001) surface: a density functional theory study.

  Oleksandr Voznyy, Jan J Dubowski

  The journal of physical chemistry. B. 12/2006; 110(47):23619-22.

  Chemisorption of alkanethiols on As-rich GaAs (001) surface under a low coverage condition was studied using first principles density functional calculations in a periodic supercell approach. The thiolate adsorption site, tilt angle and its direction are dictated by the high directionality of As dan... [more] Chemisorption of alkanethiols on As-rich GaAs (001) surface under a low coverage condition was studied using first principles density functional calculations in a periodic supercell approach. The thiolate adsorption site, tilt angle and its direction are dictated by the high directionality of As dangling bond and sulfur 3p orbital participating in bonding and steric repulsion of the first three CH2 units from the surface. Small charge transfer between thiolate and surface, strong dependence of total energy on tilt angle, and a relatively short length of 2.28 A of the S-As bond indicate the highly covalent nature of the bonding. Calculated binding energy of 2.1 eV is consistent with the available experimental data.

• Multibandgap quantum well wafers by IR laser quantum well intermixing : simulation of the lateral resolution of the process

  Oleksandr Voznyy, Radoslaw Stanowski, Jan J. Dubowski

  Journal of Laser Micro/Nanoengineering. 01/2006; 1:48-48.

  Post-growth selective-area bandgap tuning of quantum well (QW) microstructures has been investigated using Finite Element Method computer simulations. Laser fast scanning is proposed as a way to overcome the problem of damaging the surface with a small spot needed to obtain better spatial resolution... [more] Post-growth selective-area bandgap tuning of quantum well (QW) microstructures has been investigated using Finite Element Method computer simulations. Laser fast scanning is proposed as a way to overcome the problem of damaging the surface with a small spot needed to obtain better spatial resolution. Influence of laser parameters, background heating, beam scanning speed and properties of QW microstructures on the ability to achieve sharp bandgap profiles in the laser- written intermixed material are investigated. Lateral resolution of 2 µm is expected to be achievable with a 12-µm beam laser rapid thermal annealing.

• Thermodynamic Stability and Redistribution of Charges in Ternary AlGaN , InGaN , and InAlN Alloys

  V G Deibuk, Oleksandr Voznyy

  Semiconductors. 01/2005; 39:623-628.

  A model of the delta lattice parameter is used to study the thermodynamics of AlGaN, InGaN, and InAlN alloys. The phase diagrams obtained indicate that Al x Ga 1 – x N is stable in the entire range of x , whereas the miscibility gap corresponds to 0.2 < x < 0.69 for In x Ga 1 – x N and to 0.16... [more] A model of the delta lattice parameter is used to study the thermodynamics of AlGaN, InGaN, and InAlN alloys. The phase diagrams obtained indicate that Al x Ga 1 – x N is stable in the entire range of x , whereas the miscibility gap corresponds to 0.2 < x < 0.69 for In x Ga 1 – x N and to 0.16 < x < 0.7 for In x Al 1 – x N at 1000 K. Biaxial stresses lower the critical temperature and narrow the miscibility gap. The charge-density distribution is analyzed using the pseudopotential method to obtain an approximation of 32-atom supercells. The results of the analysis show that the stability of these alloys is controlled by the competition between the destabilizing contribution of strains related to the mismatch between the lattice constants and a stabilizing charge exchange between various chemical bonds. Biaxial stress reduces the charge redistribution caused by strains and thus increases the stability of an alloy. ©

• The Role of Alloying Effects in the Formation of Electronic Structure of Unordered Group III Nitride Solid Solutions

  Oleksandr Voznyy, V G Deibuk

  Semiconductors. 01/2004; 38:316-321.

  The effect of compositional and positional disorder on electronic properties of (Group III)-nitride solid solutions with the wurtzite structure was studied by the method of a model empirical pseudopotential using 32-atom supercells. The calculated values of the band-gap bowing parameter are found to... [more] The effect of compositional and positional disorder on electronic properties of (Group III)-nitride solid solutions with the wurtzite structure was studied by the method of a model empirical pseudopotential using 32-atom supercells. The calculated values of the band-gap bowing parameter are found to be equal to 0.44, 2.72, and 4.16 for AlGaN, InGaN, InAlN, respectively. It is shown that the major contribution to the band-gap bowing parameter is made by the compositional disorder, whereas the bond-length relaxation reduces the effect of com- positional disorder and the effects of the volume deformation.

• Gold adatom as a key structural component in self-assembled monolayers of organosulfur molecules on Au(111)

  Peter Maksymovych, Oleksandr Voznyy, Daniel B. Dougherty, Dan C. Sorescu, John T. Yates

  Progress in Surface Science.

  Chemisorption of organosulfur molecules, such as alkanethiols, arenethiols and disulfide compounds on gold surfaces and their subsequent self-organization is the archetypal process for molecular self-assembly on surfaces. Owing to their ease of preparation and high versatility, alkanethiol self-asse... [more] Chemisorption of organosulfur molecules, such as alkanethiols, arenethiols and disulfide compounds on gold surfaces and their subsequent self-organization is the archetypal process for molecular self-assembly on surfaces. Owing to their ease of preparation and high versatility, alkanethiol self-assembled monolayers (SAMs) have been widely studied for potential applications including surface functionalization, molecular motors, molecular electronics, and immobilization of biological molecules. Despite fundamental advances, the dissociative chemistry of the sulfur headgroup on gold leading to the formation of the sulfur–gold anchor bond has remained controversial. This review summarizes the recent progress in the understanding of the geometrical and electronic structure of the anchor bond. Particular attention is drawn to the involvement of gold adatoms at all stages of alkanethiol self-assembly, including the dissociation of the disulfide (S–S) and hydrogen-sulfide (S–H) bonds and subsequent formation of the self-assembled structure. Gold adatom chemistry is proposed here to be a unifying theme that explains various aspects of the alkanethiol self-assembly and reconciles experimental evidence provided by scanning probe microscopy and spectroscopic methods of surface science. While several features of alkanethiol self-assembly have yet to be revisited in light of the new adatom-based models, the successes of alkanethiol SAMs suggest that adatom-mediated surface chemistry may be a viable future approach for the construction of self-assembled monolayers involving molecules which do not contain sulfur.