Danda Acharya - Academia.edu (original) (raw)
Papers by Danda Acharya
Oxygen vacancies are among the primary chemically active defects on the surface of reducible tran... more Oxygen vacancies are among the primary chemically active defects on the surface of reducible transition metal oxides, playing a key role in surface chemistry, catalysis, and photocatalysis. We report the controlled manipulation of individual O-vacancies on reduced TiO2(110)-1x1 using a low temperature scanning tunneling microscope. Localized voltage pulses trigger the hopping of single vacancies along a bridging oxygen (Obr) row.
The Journal of Physical Chemistry C, 2011
... DP Acharya, N. Camillone, III, and P. Sutter*. Center for Functional Nanomaterials, Brookh... more ... DP Acharya, N. Camillone, III, and P. Sutter*. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States. Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States. J. Phys. Chem. ...
The nitrogen polar gallium-rich GaN (0001 ) samples are grown on sapphire (0001) substrates at sa... more The nitrogen polar gallium-rich GaN (0001 ) samples are grown on sapphire (0001) substrates at sample temperature of 650 ^0 C using r.f. N-plasma molecular beam epitaxy. During the growth, the surface is monitored by using reflection high energy electron diffraction and 3x3 and 6x6 reconstructed surface patterns have been observed. The freshly grown samples are then transferred to a
Field enhancement effects at metallic nanostructures are crucial for experimental techniques such... more Field enhancement effects at metallic nanostructures are crucial for experimental techniques such as surface-enhanced Raman spectroscopy and scanning near-field optical microscopy that detect optical signals (e.g., due to Raman scattering, surface plasmon resonances, fluorescence, or frequency mixing) to interrogate surface chemical and physical interactions at the nanoscale. However, the spatial resolution of these all- optical techniques is limited to tens
The evolution of the surface state and the effect of vacancies on the Ag(111) surface are investi... more The evolution of the surface state and the effect of vacancies on the Ag(111) surface are investigated at an atomic scale by combining scanning tunneling microscopy and spectroscopy and atom manipulation at 5 K. Various vacancy sizes, from one atom to hundreds of atoms, on Ag(111) are first created by tip-sample contact in a controlled manner. Using lateral manipulation, a vacancy is filled one at a time and the corresponding differential conductance spectra are recorded. Small energy shifts in the onset of surface state are observed. The shift is more pronounced for small size vacancy and becomes less and less pronounced for larger size vacancies. The observed dI/dV intensities at different size of vacancies clearly reveals that the surface state onset on Ag(111) disappears after reaching towards the 6^th layers.
Aps March Meeting Abstracts, 2006
We investigate the vacancy electronic structures on a Ag(111) surface at 4.6 K by using scanning-... more We investigate the vacancy electronic structures on a Ag(111) surface at 4.6 K by using scanning-tunneling-microscopy (STM) manipulation and spectroscopy in an ultra-high-vacuum environment. The vacancies with single atom to over a hundred-atom sizes are created on atomically cleaned Ag(111) surface by controlled tip-sample contact and single atom manipulation. Conductance tunneling spectroscopy data are then acquired by using a lock-in amplifier attached to the STM. The shift of the on-set of Ag(111) surface state is observed in few-atom vacancies. For the larger vacancies, we are able to probe the bulk silver unoccupied s-p band using tunneling spectroscopy. This experiment demonstrates a unique capability of a combined STM manipulation and spectroscopy to probe bulk and surface properties of materials through engineered atom and nanoscale cavities. This work is financially supported by a US-DOE grant, DE-FG02-02ER46012, and a NSF-NIRT grant, DMR-0304314.
Using low-temperature scanning tunneling microscopy, we demonstrate an unambiguous 1-D system tha... more Using low-temperature scanning tunneling microscopy, we demonstrate an unambiguous 1-D system that surprisingly undergoes a CDW instability on a metallic substrate. Our ability to directly and quantitatively measure the structural distortion of this system provides an accurate reference for comparison with first principles theory. In comparison to previously proposed physical mechanisms, we attribute this particular 1-D CDW instability to a ferromagnetic state. We show that though the linear arrayed dimers are not electronically isolated, they are magnetically independent, and hence can potentially serve as a binary spin-memory system.
Using density functional theory calculations at the level of Hubbard-corrected generalized gradie... more Using density functional theory calculations at the level of Hubbard-corrected generalized gradient approximation (GGA+ U), we calculate the formation and interaction energies of oxygen vacancies on the (110) surface of rutile for neutral and positively charged slabs for ...
An ultra-high-vacuum low-temperature scanning-tunneling-microscope (UHV-LT-STM) system capable of... more An ultra-high-vacuum low-temperature scanning-tunneling-microscope (UHV-LT-STM) system capable of performing cutting-edge single atom/molecule manipulations on molecular-beam-epitaxy (MBE) grown nitride surfaces has been designed and constructed. A modified Beasoke-Beetle type scanner is used for the STM and it is designed to operate at liquid helium and liquid nitrogen temperatures. The STM unit is directly attached to a UHV bath-cryostat allowing cooling down to 5 K surface temperature. The thermal drift of our STM at liquid helium temperature is less than 0.1nm/hr and hence it is possible to position the STM-tip over a medium size molecule for a long time. In addition to the manipulation, our system allows single atom/molecule level tunneling spectroscopy measurements such as I/V, dI-dV/V, d2I-dV2/V (vibrational STS) and dI-dZ/V schemes. The UHV system also include Ne ion sputtering facility for sample cleaning, a sample manipulator with X,Y,Z and rotational stages as well as sam...
Single stranded DNA (ssDNA) interacts strongly with carbon nanotubes (CNTs) to form a stable DNA-... more Single stranded DNA (ssDNA) interacts strongly with carbon nanotubes (CNTs) to form a stable DNA-CNT hybrid that effectively disperses CNTs in aqueous solution. In our experiment, ss-DNA oligonucleotide with a repeating G-T sequence was used. Atomic Force Microscopy and Low Temperature Scanning Tunneling Microscopy are used to image the structures of DNA-CNT on gold substrate. Our experimental result shows an
Due to stronger electron-electron interactions, 1-D systems are predicted and, in some cases, hav... more Due to stronger electron-electron interactions, 1-D systems are predicted and, in some cases, have been shown to exhibit unique and exotic electronic properties. One route to the formation of 1-D systems is by self-assembly using low-index vicinal crystal surfaces. In this regard, we have successfully formed 1-atom wide Co wires using Cu(775), a 7-atom wide stepped array with (111) terraces.
We recently reported [1] on a new surface phase of the Co-vicinal-Cu(111) system which exhibits s... more We recently reported [1] on a new surface phase of the Co-vicinal-Cu(111) system which exhibits self-assembled uniform Co quantum wires that are stable at 300K.STM images show that the wires form along the leading edge of the step rise, differentiating it from previously theoretically predicted atomic-wire phases as well as experimentally observed step-island formation. Our observations allow us to comment
We recently reported [1] on a new surface phase of the Co-vicinal-Cu(111) system, which exhibits ... more We recently reported [1] on a new surface phase of the Co-vicinal-Cu(111) system, which exhibits self-assembled uniform Co quantum wires that are stable at 300K. STM images show that the wires form along the leading edge of the step rise, differentiating it from previously theoretically predicted atomic-wire phases as well as experimentally observed step-island formation. Our observations allow us to
ABSTRACT Field enhancement effects at metallic nanostructures are crucial for experimental techni... more ABSTRACT Field enhancement effects at metallic nanostructures are crucial for experimental techniques such as surface-enhanced Raman spectroscopy and scanning near-field optical microscopy that detect optical signals (e.g., due to Raman scattering, surface plasmon resonances, fluorescence, or frequency mixing) to interrogate surface chemical and physical interactions at the nanoscale. However, the spatial resolution of these all- optical techniques is limited to tens of nanometers due to the low photon yield from small sample volumes. We report on a new approach that uses the probe tip of a scanning tunneling microscope to detect electrons emitted during time-resolved ultrafast laser excitation of a metal surface. We show how we have used this technique to image two-photon field enhancements near the surfaces of single nanoparticles with sub-nanometer spatial resolution. We also describe how this approach may enable the simultaneous sub-nanometer spatial and sub- picosecond temporal resolution of surface electron dynamics.
We demonstrate an atomic scale construction scheme, which is performed at an area as small as a f... more We demonstrate an atomic scale construction scheme, which is performed at an area as small as a few tens of nanometer square. In this atomic scale construction site, all the basic building blocks, single atoms, are extracted locally using a scanning-tunneling-microscope tip from the substrate. These extracted atoms are then precisely positioned on the surface to form desired structures. After the completion of the construction, the remaining debris are removed and the undesired holes near the construction site are filled with atoms/clusters to tidy up the area. This entire construction scheme closely resembles our real world construction process and can be considered as its atomic-scale analog.
The Journal of Physical Chemistry C, 2012
ABSTRACT The photoactivity of methanol on the rutile TiO2(110) surface is shown to depend on the ... more ABSTRACT The photoactivity of methanol on the rutile TiO2(110) surface is shown to depend on the ability of methanol to diffuse on the surface and find sites active for its thermal dissociation to methoxy. Temperature programmed desorption (TPD) results show that the extent of methanol photodecomposition to formaldehyde is negligible on the clean TiO2(110) surface at 100 K due to a scarcity of sites that can convert (photoinactive) methanol to (photoactive) methoxy. The extent of photoactivity at 100 K significantly increases when methanol is coadsorbed with oxygen, however only those molecules able to adsorb near (next to) a coadsorbed oxygen species are active. Preannealing coadsorbed methanol and oxygen to above 200 K prior to UV irradiation results in a significant increase in photoactivity. Scanning tunneling microscopy (STM) images clearly show that the advent of increased photoactivity in TPD correlates with the onset of methanol diffusion along the surface’s Ti4+ rows at ~200 K. These results demonstrate that optimizing thermal processes (such as diffusion or proton transfer reactions) can be critical to maximizing photocatalytic reactivity on TiO2 surfaces. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle under contract DEAC05-76RL01830. The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.
The Journal of Physical Chemistry C, 2013
ABSTRACT Two-dimensional Au islands of different thicknesses grown on graphene/Ru(0001) were used... more ABSTRACT Two-dimensional Au islands of different thicknesses grown on graphene/Ru(0001) were used to study the corrugation of the moiré structure of graphene/Ru(0001) and discriminate between its mainly structural or electronic character. A comparison of the apparent corrugation measured by scanning tunneling microscopy (STM) for different Au thicknesses with results of elasticity theory equations applied to a gold film over a corrugated substrate shows that the corrugation observed for the graphene/Ru(0001) moiré is of structural nature rather than electronic. STM showed a large value for the corrugation of the first Au monolayer on graphene/Ru(0001), 1.7 Å; using density functional theory calculations, we explain this large corrugation of the Au monolayer as the result of a strong (weak) binding of the Au layer at the valley (hill) regions of the graphene/Ru(0001) moiré structure and infer an actual corrugation of the graphene/Ru(0001) moiré structure of 1.2 Å from the measured corrugation of the Au monolayer.
The Journal of Physical Chemistry C, 2011
... is estimated to be from 0.5–0.8 eV depending on the details of the calculation (eg, functiona... more ... is estimated to be from 0.5–0.8 eV depending on the details of the calculation (eg, functional used), which ... for vertically bonded PDI are located at 0.7 and 3 eV above E F , respectively, fora PDI coverage of 0.33 ML.(16) An analysis of the projected density of states ...
Oxygen vacancies are among the primary chemically active defects on the surface of reducible tran... more Oxygen vacancies are among the primary chemically active defects on the surface of reducible transition metal oxides, playing a key role in surface chemistry, catalysis, and photocatalysis. We report the controlled manipulation of individual O-vacancies on reduced TiO2(110)-1x1 using a low temperature scanning tunneling microscope. Localized voltage pulses trigger the hopping of single vacancies along a bridging oxygen (Obr) row.
The Journal of Physical Chemistry C, 2011
... DP Acharya, N. Camillone, III, and P. Sutter*. Center for Functional Nanomaterials, Brookh... more ... DP Acharya, N. Camillone, III, and P. Sutter*. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States. Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States. J. Phys. Chem. ...
The nitrogen polar gallium-rich GaN (0001 ) samples are grown on sapphire (0001) substrates at sa... more The nitrogen polar gallium-rich GaN (0001 ) samples are grown on sapphire (0001) substrates at sample temperature of 650 ^0 C using r.f. N-plasma molecular beam epitaxy. During the growth, the surface is monitored by using reflection high energy electron diffraction and 3x3 and 6x6 reconstructed surface patterns have been observed. The freshly grown samples are then transferred to a
Field enhancement effects at metallic nanostructures are crucial for experimental techniques such... more Field enhancement effects at metallic nanostructures are crucial for experimental techniques such as surface-enhanced Raman spectroscopy and scanning near-field optical microscopy that detect optical signals (e.g., due to Raman scattering, surface plasmon resonances, fluorescence, or frequency mixing) to interrogate surface chemical and physical interactions at the nanoscale. However, the spatial resolution of these all- optical techniques is limited to tens
The evolution of the surface state and the effect of vacancies on the Ag(111) surface are investi... more The evolution of the surface state and the effect of vacancies on the Ag(111) surface are investigated at an atomic scale by combining scanning tunneling microscopy and spectroscopy and atom manipulation at 5 K. Various vacancy sizes, from one atom to hundreds of atoms, on Ag(111) are first created by tip-sample contact in a controlled manner. Using lateral manipulation, a vacancy is filled one at a time and the corresponding differential conductance spectra are recorded. Small energy shifts in the onset of surface state are observed. The shift is more pronounced for small size vacancy and becomes less and less pronounced for larger size vacancies. The observed dI/dV intensities at different size of vacancies clearly reveals that the surface state onset on Ag(111) disappears after reaching towards the 6^th layers.
Aps March Meeting Abstracts, 2006
We investigate the vacancy electronic structures on a Ag(111) surface at 4.6 K by using scanning-... more We investigate the vacancy electronic structures on a Ag(111) surface at 4.6 K by using scanning-tunneling-microscopy (STM) manipulation and spectroscopy in an ultra-high-vacuum environment. The vacancies with single atom to over a hundred-atom sizes are created on atomically cleaned Ag(111) surface by controlled tip-sample contact and single atom manipulation. Conductance tunneling spectroscopy data are then acquired by using a lock-in amplifier attached to the STM. The shift of the on-set of Ag(111) surface state is observed in few-atom vacancies. For the larger vacancies, we are able to probe the bulk silver unoccupied s-p band using tunneling spectroscopy. This experiment demonstrates a unique capability of a combined STM manipulation and spectroscopy to probe bulk and surface properties of materials through engineered atom and nanoscale cavities. This work is financially supported by a US-DOE grant, DE-FG02-02ER46012, and a NSF-NIRT grant, DMR-0304314.
Using low-temperature scanning tunneling microscopy, we demonstrate an unambiguous 1-D system tha... more Using low-temperature scanning tunneling microscopy, we demonstrate an unambiguous 1-D system that surprisingly undergoes a CDW instability on a metallic substrate. Our ability to directly and quantitatively measure the structural distortion of this system provides an accurate reference for comparison with first principles theory. In comparison to previously proposed physical mechanisms, we attribute this particular 1-D CDW instability to a ferromagnetic state. We show that though the linear arrayed dimers are not electronically isolated, they are magnetically independent, and hence can potentially serve as a binary spin-memory system.
Using density functional theory calculations at the level of Hubbard-corrected generalized gradie... more Using density functional theory calculations at the level of Hubbard-corrected generalized gradient approximation (GGA+ U), we calculate the formation and interaction energies of oxygen vacancies on the (110) surface of rutile for neutral and positively charged slabs for ...
An ultra-high-vacuum low-temperature scanning-tunneling-microscope (UHV-LT-STM) system capable of... more An ultra-high-vacuum low-temperature scanning-tunneling-microscope (UHV-LT-STM) system capable of performing cutting-edge single atom/molecule manipulations on molecular-beam-epitaxy (MBE) grown nitride surfaces has been designed and constructed. A modified Beasoke-Beetle type scanner is used for the STM and it is designed to operate at liquid helium and liquid nitrogen temperatures. The STM unit is directly attached to a UHV bath-cryostat allowing cooling down to 5 K surface temperature. The thermal drift of our STM at liquid helium temperature is less than 0.1nm/hr and hence it is possible to position the STM-tip over a medium size molecule for a long time. In addition to the manipulation, our system allows single atom/molecule level tunneling spectroscopy measurements such as I/V, dI-dV/V, d2I-dV2/V (vibrational STS) and dI-dZ/V schemes. The UHV system also include Ne ion sputtering facility for sample cleaning, a sample manipulator with X,Y,Z and rotational stages as well as sam...
Single stranded DNA (ssDNA) interacts strongly with carbon nanotubes (CNTs) to form a stable DNA-... more Single stranded DNA (ssDNA) interacts strongly with carbon nanotubes (CNTs) to form a stable DNA-CNT hybrid that effectively disperses CNTs in aqueous solution. In our experiment, ss-DNA oligonucleotide with a repeating G-T sequence was used. Atomic Force Microscopy and Low Temperature Scanning Tunneling Microscopy are used to image the structures of DNA-CNT on gold substrate. Our experimental result shows an
Due to stronger electron-electron interactions, 1-D systems are predicted and, in some cases, hav... more Due to stronger electron-electron interactions, 1-D systems are predicted and, in some cases, have been shown to exhibit unique and exotic electronic properties. One route to the formation of 1-D systems is by self-assembly using low-index vicinal crystal surfaces. In this regard, we have successfully formed 1-atom wide Co wires using Cu(775), a 7-atom wide stepped array with (111) terraces.
We recently reported [1] on a new surface phase of the Co-vicinal-Cu(111) system which exhibits s... more We recently reported [1] on a new surface phase of the Co-vicinal-Cu(111) system which exhibits self-assembled uniform Co quantum wires that are stable at 300K.STM images show that the wires form along the leading edge of the step rise, differentiating it from previously theoretically predicted atomic-wire phases as well as experimentally observed step-island formation. Our observations allow us to comment
We recently reported [1] on a new surface phase of the Co-vicinal-Cu(111) system, which exhibits ... more We recently reported [1] on a new surface phase of the Co-vicinal-Cu(111) system, which exhibits self-assembled uniform Co quantum wires that are stable at 300K. STM images show that the wires form along the leading edge of the step rise, differentiating it from previously theoretically predicted atomic-wire phases as well as experimentally observed step-island formation. Our observations allow us to
ABSTRACT Field enhancement effects at metallic nanostructures are crucial for experimental techni... more ABSTRACT Field enhancement effects at metallic nanostructures are crucial for experimental techniques such as surface-enhanced Raman spectroscopy and scanning near-field optical microscopy that detect optical signals (e.g., due to Raman scattering, surface plasmon resonances, fluorescence, or frequency mixing) to interrogate surface chemical and physical interactions at the nanoscale. However, the spatial resolution of these all- optical techniques is limited to tens of nanometers due to the low photon yield from small sample volumes. We report on a new approach that uses the probe tip of a scanning tunneling microscope to detect electrons emitted during time-resolved ultrafast laser excitation of a metal surface. We show how we have used this technique to image two-photon field enhancements near the surfaces of single nanoparticles with sub-nanometer spatial resolution. We also describe how this approach may enable the simultaneous sub-nanometer spatial and sub- picosecond temporal resolution of surface electron dynamics.
We demonstrate an atomic scale construction scheme, which is performed at an area as small as a f... more We demonstrate an atomic scale construction scheme, which is performed at an area as small as a few tens of nanometer square. In this atomic scale construction site, all the basic building blocks, single atoms, are extracted locally using a scanning-tunneling-microscope tip from the substrate. These extracted atoms are then precisely positioned on the surface to form desired structures. After the completion of the construction, the remaining debris are removed and the undesired holes near the construction site are filled with atoms/clusters to tidy up the area. This entire construction scheme closely resembles our real world construction process and can be considered as its atomic-scale analog.
The Journal of Physical Chemistry C, 2012
ABSTRACT The photoactivity of methanol on the rutile TiO2(110) surface is shown to depend on the ... more ABSTRACT The photoactivity of methanol on the rutile TiO2(110) surface is shown to depend on the ability of methanol to diffuse on the surface and find sites active for its thermal dissociation to methoxy. Temperature programmed desorption (TPD) results show that the extent of methanol photodecomposition to formaldehyde is negligible on the clean TiO2(110) surface at 100 K due to a scarcity of sites that can convert (photoinactive) methanol to (photoactive) methoxy. The extent of photoactivity at 100 K significantly increases when methanol is coadsorbed with oxygen, however only those molecules able to adsorb near (next to) a coadsorbed oxygen species are active. Preannealing coadsorbed methanol and oxygen to above 200 K prior to UV irradiation results in a significant increase in photoactivity. Scanning tunneling microscopy (STM) images clearly show that the advent of increased photoactivity in TPD correlates with the onset of methanol diffusion along the surface’s Ti4+ rows at ~200 K. These results demonstrate that optimizing thermal processes (such as diffusion or proton transfer reactions) can be critical to maximizing photocatalytic reactivity on TiO2 surfaces. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle under contract DEAC05-76RL01830. The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.
The Journal of Physical Chemistry C, 2013
ABSTRACT Two-dimensional Au islands of different thicknesses grown on graphene/Ru(0001) were used... more ABSTRACT Two-dimensional Au islands of different thicknesses grown on graphene/Ru(0001) were used to study the corrugation of the moiré structure of graphene/Ru(0001) and discriminate between its mainly structural or electronic character. A comparison of the apparent corrugation measured by scanning tunneling microscopy (STM) for different Au thicknesses with results of elasticity theory equations applied to a gold film over a corrugated substrate shows that the corrugation observed for the graphene/Ru(0001) moiré is of structural nature rather than electronic. STM showed a large value for the corrugation of the first Au monolayer on graphene/Ru(0001), 1.7 Å; using density functional theory calculations, we explain this large corrugation of the Au monolayer as the result of a strong (weak) binding of the Au layer at the valley (hill) regions of the graphene/Ru(0001) moiré structure and infer an actual corrugation of the graphene/Ru(0001) moiré structure of 1.2 Å from the measured corrugation of the Au monolayer.
The Journal of Physical Chemistry C, 2011
... is estimated to be from 0.5–0.8 eV depending on the details of the calculation (eg, functiona... more ... is estimated to be from 0.5–0.8 eV depending on the details of the calculation (eg, functional used), which ... for vertically bonded PDI are located at 0.7 and 3 eV above E F , respectively, fora PDI coverage of 0.33 ML.(16) An analysis of the projected density of states ...