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AFRL-AFOSR-VA-TR-2016-0264 


MICROSCOPIC ELECTRONIC AND MECHANICAL 
PROPERTIES OF ULTRA-THIN LAYERED MATERIALS 


Abhay Pasupathy 

THE TRUSTEES OF COLUMBIA UNIVERSITY IN THE CITY OF NEW YORK 
116TH AND BDWY 
NEW YORK, NY 10027 


07/25/2016 
Final Report 


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MICROSCOPIC ELECTRONIC AND MECHANICAL PROPERTIES OF ULTRA-THIN LAYERED 
MATERIALS 

5a. CONTRACTNUMBER 

5b. GRANTNUMBER 

FA9550-11-1-0010 

5c. PROGRAM ELEMENTNUMBER 

61102F 

6. AUTHORfS) 

Abhay Pasupathy 

5d. PROJ ECTNUMBER 

5e. TASK NUMBER 

5f. WORKUNTTNUMBER 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESSES) 

THE TRUSTEES O F C O LUM BIA UN IVERSITY IN THE CITY O F N EW YO RK 

116TH AND BDWY 

NEW YORK, NY 10027 US 

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AFOffice ofScientific Research 

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Arlington, VA 22203 

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AFRL/AFOSRRTB1 

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A FRL-A FO SR-VA-TR-2016-0264 

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14. ABSTRACT 

The research goalsofthisproject were to characterize the microscopic electronic and structural properties 
of ultra-thin (few layer) crystalline materia Is, commonly referred to as 2D materials. The technical approach 
used wasto perform atomic-resolution scanning tunneling microscopy a long with na nofabrication and 
transport measurements. The materia Is studied underthe grantwere graphene and transition metal 
dichalcogenides. In graphene, stud iesc entered on material created by the chemical vapordeposition 
technique. A numberof studies we re conducted to determine the optimal conditio ns to grow graphene 
under. The doping of gra phene with foreign a toms was investigated extensively. Finally, exotic electronic 
states in graphene that can be induced via substrate and adatom interactions we re investigated. In the 
transition-metal dichalcogenides, studies initially foe used on the charge density wave materials. It was 
discovered thatthe charge density wave transition is extremely sensitive to disorder. It wasthen found that 
the charge densitywave isnotdriven byFermi surface nesting, but rather by strong electron-phonon 
coupling. Superconductivity in transition metal dichalcogenides was then investigated, and it wasfound 
that in the limit of thin samples, a new Bose metal state emerges in the presence of a magnetic field. 

Finally, semiconducting transition metaldichalcogenidesgrown by metal organic chemicalvapor 
deposition were studied and theiratomic and electronic quality were measured by scanning tunneling 
microscopy. 


15. SUB| ECTIERMS 

Scanning Tunneling M ic rose opy, G ra phene 


Standard Form 298 (Rev. 8/98 
Prescribed by ANSI Std. 239.11 


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3 • REPORT 

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Abhay Pasupathy Narayan: Final Report 

Title: MICROSCOPIC ELECTRONIC AND MECHANICAL PROPERTIES OF ULTRA-THIN 

LAYERED MATERIALS 

Contract No.: FA9550S US IS 0010 

Abstract 

The research goals of this project were to characterize the microscopic electronic 
and structural properties of ultra-thin (few layer) crystalline materials, commonly referred 
to as 2D materials. The technical approach used was to perform atomic-resolution 
scanning tunneling microscopy along with nanofabrication and transport measurements. 
The materials studied under the grant were graphene and transition metal 
dichalcogenides. In graphene, studies centered on material created by the chemical vapor 
deposition technique. A number of studies were conducted to determine the optimal 
conditions to grow graphene under. The doping of graphene with foreign atoms was 
investigated extensively. Finally, exotic electronic states in graphene that can be induced 
via substrate and adatom interactions were investigated. In the transition-metal 
dichalcogenides, studies initially focused on the charge density wave materials. It was 
discovered that the charge density wave transition is extremely sensitive to disorder. It 
was then found that the charge density wave is not driven by Fermi surface nesting, but 
rather by strong electron-phonon coupling. Superconductivity in transition metal 
dichalcogenides was then investigated, and it was found that in the limit of thin samples, 
a new Bose metal state emerges in the presence of a magnetic field. Finally, 
semiconducting transition metal dichalcogenides grown by metal organic chemical vapor 
deposition were studied and their atomic and electronic quality were measured by 
scanning tunneling microscopy. 

Description 

The primary goal of this project was to characterize the structural and electronic 
quality of new two-dimensional (2D) materials that were grown by chemical vapor 
deposition (CVD) on metal and insulator surfaces. Under this theme, studies were 
performed on pristine graphene, doped graphene and transition-metal dichalcogenides. 
Apart from this, studies were also performed on exfoliated samples, primarily focusing 
on materials beyond graphene to study new electronic functionality at the ultrathin limit. 
A total of 18 publications and 4 in review/press have resulted from the award - one in 
Science, four in Nature Physics and two in Phys Rev Letters. Two patents have been filed 
based on the award. Described below are the summaries of each of these works: 

1. Growth of graphene on single crystal copper substrates (reference [11): 

We study the influence of the surface structure of copper single crystals on the growth of 
large area monolayer graphene by chemical vapor deposition (CVD) in ultra-high 
vacuum (UHV). Using atomic resolution scanning tunneling microscopy (STM), we find 
that graphene grows primarily in registry with the underlying copper lattice for both 
Cu(lll) and Cu(100). The graphene has a hexagonal superstructure on Cu(lll) with a 
significant electronic component,whereas it has a linear superstructure on Cu(100). 
Graphene on Cu(lll) forms a microscopically uniform sheet, the quality of which is 
detennined by the presence of grain boundaries where graphene grains with different 


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orientations meet. Graphene grown on Cu(100) under similar conditions does not form a 
uniform sheet and instead displays exposed nanoscale edges. Our results indicate the 
importance of the copper crystal structure on the microstructure of graphene films 
produced by CVD. 

2. Visualizing Individual Nitrogen Dopants in Monolayer Graphene (reference [21) 

In monolayer graphene, substitutional doping during growth can be used to alter its 
electronic properties. We used scanning tunneling microscopy (STM), Raman 
spectroscopy, x-ray spectroscopy, and first principles calculations to characterize 
individual nitrogen dopants in monolayer graphene grown on a copper substrate. 
Individual nitrogen atoms were incorporated as graphitic dopants, and a fraction of the 
extra electron on each nitrogen atom was delocalized into the graphene lattice. The 
electronic structure of nitrogen-doped graphene was strongly modified only within a few 
lattice spacings of the site of the nitrogen dopant. These findings show that chemical 
doping is a promising route to achieving high-quality graphene films with a large carrier 
concentration. 

3. Large phvsisorption strain in chemical vapor deposition of graphene on copper 
substrates (reference f31) 

Graphene single layers grown by chemical vapor deposition on single crystal Cu 
substrates are subject to nonuniform physisorption strains that depend on the orientation 
of the Cu surface. The strains are revealed in Raman spectra and quantitatively 
interpreted by molecular dynamics (MD) simulations. An average compressive strain on 
the order of 0.5% is detennined in graphene on Cu (111). In graphene on Cu (100), MD 
simulations interpret the observed highly nonuniform strains. 

4. Connecting dopant bond type with electronic structure in N-doped graphene (reference 

L4J) 

Robust methods to tune the unique electronic properties of graphene by chemical 
modification are in great demand due to the potential of the two dimensional material 
to impact a range of device applications. Here we show that carbon and nitrogen core¬ 
level resonant X-ray spectroscopy is a sensitive probe of chemical bonding and electronic 
structure of chemical dopants introduced in single-sheet graphene films. In 
conjunction with density functional theory based calculations, we are able to obtain a 
detailed picture of bond types and electronic structure in graphene doped with nitrogen at 
the sub-percent level. We show that different N-bond types, including graphitic, 
pyridinic, and nitrilic, can exist in a single, dilutely N-doped graphene sheet. We show 
that these various bond types have profoundly different effects on the carrier 
concentration, indicating that control over the dopant bond type is a crucial requirement 
in advancing graphene electronics. 

5. Molecular beam growth o f graphene nanocrystals on dielectric substrates (re ference 

m 

We demonstrate the growth of graphene nanocrystals by molecular beam methods that 
employ a solid carbon source, and that can be used on a diverse class of large area 
dielectric substrates. Characterization by Raman and Near Edge X-ray Absorption Fine 


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Structure spectroscopies reveal a sp2 hybridized hexagonal carbon lattice in the 
nanocrystals. Lower growth rates favor the fonnation of higher quality, larger size multi¬ 
layer graphene crystallites on all investigated substrates. The surface morphology is 
detennined by the roughness of the underlying substrate and graphitic monolayer steps 
are observed by ambient scanning tunneling microscopy. 

6. Substrate level control of the local doping in graphene (reference [61) 

Graphene exfoliated onto muscovite mica is studied using ultrahigh vacuum scanning 
tunneling microscopy (UHV-STM) techniques. Mica provides an interesting dielectric 
substrate interface to measure the properties of graphene due to the ultraflat nature of a 
cleaved mica surface and the surface electric dipoles it possesses. Flat regions of the mica 
surface show some surface modulation of the graphene topography (24 pm) due to 
topographic modulation of the mica surface and full confonnation of the graphene to that 
surface. In addition to these ultraflat regions, plateaus of varying size having been found. 
A comparison of topographic images and STS measurements show that these plateaus are 
of two types: one with characteristics of water monolayer formation between the 
graphene and mica, and the other arising from potassium ions trapped at the interfacial 
region. Immediately above the water induced plateaus, graphene is insulated from charge 
doping, while p-type doping is observed in areas adjacent to these water nucleation 
points. However, above and in the neighborhood of interfacial potassium ions, only n- 
type doping is observed. Graphene regions above the potassium ions are more strongly n- 
doped than regions adjacent to these alkali atom plateaus. Furthermore, a direct 
correlation of these Fenni level shifts with topographic features is seen without the 
random charge carrier density modulation observed in other dielectric substrates. This 
suggests a possible route to nanoscopic control of the local electron and hole doping in 
graphene via specific substrate architecture. 

7. Visualizing the Charge Density Wave Transition in 2H-NbSei in Real Space (reference 

mi 

We report the direct observation in real space of the charge density wave (CDW) phase 
transition in pristine 2H-NbSe2 using atomic-resolution scanning tunneling microscopy. 
We find that static CDW order is established in nanoscale regions in the vicinity of 
defects at temperatures that are several times the bulk transition temperature TCDW. On 
lowering the temperature, the correlation length of these patches increases steadily until 
CDW order is established in all of space, demonstrating the crucial role played by defects 
in the physics of the transition region. The nanoscale CDW order has an energy- and 
temperature-independent wavelength. Spectroscopic imaging measurements of the real- 
space phase of the CDW provide indirect evidence that an energy gap in NbSe 2 occurs at 
0.7 eV below the Fermi energy in the CDW phase, suggesting that strong electron-lattice 
interactions, and not Fenni surface physics, are the dominant cause for CDW formation 
in NbSe2. 

8. Local atomic and electronic structure of boron chemical doping in monolayer 
graphene (reference [81) 

We use scanning tunneling microscopy and X-ray spectroscopy to characterize the 
atomic and electronic structure of boron-doped and nitrogen-doped graphene created by 


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chemical vapor deposition on copper substrates. Microscopic measurements show that 
boron, like nitrogen, incorporates into the carbon lattice primarily in the graphitic form 
and contributes ~0.5 carriers into the graphene sheet per dopant. Density functional 
theory calculations indicate that boron dopants interact strongly with the underlying 
copper substrate while nitrogen dopants do not. The local bonding differences between 
graphitic boron and nitrogen dopants lead to large scale differences in dopant 
distribution. The distribution of dopants is observed to be completely random in the case 
of boron, while nitrogen displays strong sublattice clustering. Structurally, nitrogen- 
doped graphene is relatively defect-free while boron-doped graphene films show a large 
number of Stone-Wales defects. These defects create local electronic resonances and 
cause electronic scattering, but do not electronically dope the graphene film. 

9. Visualization o f electron nematicitv and unidirectional antiferroic fluctuations at high 
temperatures in NaFeAs (reference [91) 

The driving forces behind electronic nematicity in the iron pnictides remain hotly 
debated. We use atomic-resolution variable-temperature scanning tunneling spectroscopy 
to provide the first direct visual evidence that local electronic nematicity and 
unidirectional antiferroic (stripe) fluctuations persist to temperatures almost twice the 
nominal structural ordering temperature in the parent pnictide NaFeAs. Low-temperature 
spectroscopic imaging of nematically-ordered NaFeAs shows anisotropic electronic 
features that are not observed for isostructural, non-nematic LiFeAs. The local electronic 
features are shown to arise from scattering interference around crystalline defects in 
NaFeAs, and their spatial anisotropy is a direct consequence of the structural and stripe- 
magnetic order present at low temperature. We show that the anisotropic features persist 
up to high temperatures in the nominally tetragonal phase of the crystal. The spatial 
distribution and energy dependence of the anisotropy at high temperatures is explained by 
the persistence of large amplitude, short-range, unidirectional, antiferroic (stripe) 
fluctuations, indicating that strong density wave fluctuations exist and couple to near- 
Fermi surface electrons even far from the structural and density wave phase boundaries. 

10. Segregation of sublattice domains in nitrogen-doped graphene (reference [10]) 
Atomic-level details of dopant distributions can significantly influence the material 
properties. Using scanning tunneling microscopy, we investigate the distribution of 
substitutional dopants in nitrogen-doped graphene with regard to sublattice occupancy 
within the honeycomb structure. Samples prepared by chemical vapor deposition (CVD) 
using pyridine on copper exhibit well-segregated domains of nitrogen dopants in the 
same sublattice, extending beyond 100 mn. On the other hand, samples prepared by 
postsynthesis doping of pristine graphene exhibit a random distribution between 
sublattices. On the basis of theoretical calculations, we attribute the formation of 
sublattice domains to the preferential attachment of nitrogen to the edge sites of graphene 
during the CVD growth process. The breaking of sublattice symmetry in doped graphene 
can have important implications in its electronic applications, such as the opening of a 
tunable band gap in the material. 

11. Experimental evidence for a Brass glass density wave phase in a transition-metal 
dichalcogenide preference fill) 


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Analysis of the spatial dependence of current-voltage characteristics obtained from 
scanning tunneling microscopy experiments indicates that the charge density wave 
(CDW) occurring in NbSe 2 is subject to locally strong pinning by a non-negligible 
density of defects, but that on the length scales accessible in this experiment the material 
is in a “Bragg glass” phase where dislocations and antidislocations occur in bound pairs 
and free dislocations are not observed. An analysis based on a Landau theory is presented 
showing how a strong local modulation may produce only a weak long range effect on 
the CDW phase. 

12. Quasiparticle Interference, Quasiparticle Interactions, and the Origin o f the Charge 
Density Wave in 2 H-NbSe ? (reference [121) 

We show that a small number of intentionally introduced defects can be used as a 
spectroscopic tool to amplify quasiparticle interference in 2H-NbSe 2 that we measure by 
scanning tunneling spectroscopic imaging. We show, from the momentum and energy 
dependence of the quasiparticle interference, that Fermi surface nesting is 
inconsequential to charge density wave formation in 2H-NbSe 2 . We demonstrate that, by 
combining quasiparticle interference data with additional knowledge of the quasiparticle 
band structure from angle resolved photoemission measurements, one can extract the 
wave vector and energy dependence of the important electronic scattering processes 
thereby obtaining direct information both about the fermiology and the interactions. In 
2H-NbSe 2 , we use this combination to confirm that the important near-Fermi-surface 
electronic physics is dominated by the coupling of the quasiparticles to soft mode 
phonons at a wave vector different from the charge density wave ordering wave vector. 

13. Nature of the quantum metal in a two-dimensional crystalline superconductor 
(reference [13]) 

Two-dimensional (2D) materials are not expected to be metals at low temperature owing 
to electron localization. Consistent with this, pioneering studies on thin films reported 
only superconducting and insulating ground states, with a direct transition between the 
two as a function of disorder or magnetic field. However, more recent works have 
revealed the presence of an intermediate quantum metallic state occupying a substantial 
region of the phase diagram, whose nature is intensely debated. Here, we observe such a 
state in the disorder-free limit of a crystalline 2D superconductor, produced by 
mechanical co-lamination of NbSe 2 in an inert atmosphere. Under a small perpendicular 
magnetic field, we induce a transition from superconductor to the quantum metal. We 
find a unique power-law scaling with field in this phase, which is consistent with the 
Bose-metal model where metallic behaviour arises from strong phase fluctuations caused 
by the magnetic field 

14. Structure and control of charge density waves in two-dimensional lT-TaS ? (reference 

[MR 

The layered transition metal dichalcogenides host a rich collection of charge density 
wave phases in which both the conduction electrons and the atomic structure display 
translational symmetry breaking. Manipulating these complex states by purely electronic 
methods has been a long-sought scientific and technological goal. Here, we show how 
this can be achieved in lT-TaS 2 in the 2D limit. We first demonstrate that the intrinsic 


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properties of atomically thin flakes are preserved by encapsulation with hexagonal boron 
nitride in inert atmosphere. We use this facile assembly method together with 
transmission electron microscopy and transport measurements to probe the nature of the 
2D state and show that its conductance is dominated by discommensurations. The 
discommensuration structure can be precisely tuned in few-layer samples by an in-plane 
electric current, allowing continuous electrical control over the discommensuration- 
melting transition in 2D. 

15. Modification of the G-phonon mode of graphene by nitrogen doping (reference [15]) 
The effect of nitrogen doping on the phonon spectra of graphene is analyzed. In 
particular, we employ first-principles calculations and scanning Raman analysis to 
investigate the dependence of phonon frequencies in graphene on the concentration of 
nitrogen dopants. We demonstrate that the G phonon frequency shows oscillatory 
behavior as a function of nitrogen concentration. We analyze different mechanisms which 
could potentially be responsible for this behavior, such as Friedel charge oscillations 
around the localized nitrogen impurity atom, the bond length change between nitrogen 
impurity and its nearest neighbor carbon atoms, and the long-range interactions of the 
nitrogen point defects. We show that the bond length change and the long range 
interaction of point defects are possible mechanisms responsible for the oscillatory 
behavior of the G frequency as a function of nitrogen concentration. At the same time, 
Friedel charge oscillations are unlikely to contribute to this behavior. 

16. Klein tunneling and electron trapping in nanometre-scale graphene quantum dots 
(reference [161) 

Relativistic fennions that are incident on a high potential barrier can pass through 
unimpeded, a striking phenomenon termed the “Klein paradox” in quantum 
electrodynamics. Electrostatic potential barriers in graphene provide a solid-state analog 
to realize this phenomenon. Here, we use scanning tunneling microscopy to directly 
probe the transmission of electrons through sharp circular potential wells in graphene 
created by substrate engineering. We find that electrons in this geometry display quasi¬ 
bound states where the electron is trapped for a finite time before escaping via Klein 
tunneling. We show that the continuum Dirac equation can be successfully used to model 
the energies and wavefunctions of these quasi-bound states down to atomic dimensions. 
We demonstrate that by tuning the geometry of the barrier it is possible to trap particular 
energies and angular momentum states with increased efficiency, showing that atomic- 
scale electrostatic potentials can be used to engineer quantum transport through graphene. 

17. Imaging chiral symmetry breaking from Kekule bond order in graphene (reference 

117]) 

Chirality — or "handedness" — is a symmetry property crucial to fields as diverse as 
biology, chemistry, and high-energy physics. In graphene, chiral symmetry emerges 
naturally as a consequence of the carbon honeycomb lattice. This symmetry can be 
broken by interactions that couple electrons with opposite momenta in graphene. Here we 
directly visualize the formation of Kekule bond order, one such phase of broken chiral 
symmetry, in an ultraflat graphene sheet grown epitaxially on a copper substrate. We 
show that its origin lies in the interactions between individual vacancies in the copper 


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substrate that are mediated electronically by the graphene. We show that this interaction 
causes the bonds in graphene to distort, creating a phase with broken chiral symmetry. 
The Kekule ordering is robust at ambient temperature and atmospheric conditions, 
indicating that intercalated atoms may be harnessed to drive graphene and other two- 
dimensional (2D) materials towards electronically desirable and exotic collective phases. 

18. Atomic-Scale Spectroscopy of Gated Monolayer MoS? (reference f18]) 

The electronic properties of semiconducting monolayer transition-metal dichalcogenides 
can be tuned by electrostatic gate potentials. Here we report gate tunable imaging and 
spectroscopy of monolayer M 0 S 2 by atomic-resolution scanning tunneling 
microscopy/spectroscopy (STM/STS). Our measurements are performed on large-area 
samples grown by metal-organic chemical vapor deposition (MOCVD) techniques on a 
silicon oxide substrate. Topographic measurements of defect density indicate a sample 
quality comparable to single crystal M 0 S 2 . From gate voltage dependent spectroscopic 
measurements, we determine that in-gap states exist in or near the M 0 S 2 film at a density 
of 1.3 x 10 12 eW 1 cm 2 . By combining the single-particle band gap measured by STS 
with optical measurements, we estimate an exciton binding energy of 230 meV on this 
substrate, in qualitative agreement with numerical simulation. Grain boundaries are 
observed in these polycrystalline samples, which are seen to not have strong electronic 
signatures in STM imaging. 

References (published under the grant); 

1 Zhao, L., Rim, K. T., Zhou, H., He, R., Heinz, T. F., Pinczuk, A., Flynn, G. W. & 
Pasupathy, A. N. Influence of copper crystal surface on the CVD growth of 
large area monolayer graphene. Solid State Communications 151, p. 509-513, 
( 2011 ). 

2 Zhao, L. Y. et al. Visualizing Individual Nitrogen Dopants in Monolayer 
Graphene. Science 333, p. 999-1003, (2011). 

3 He, R., Zhao, L. Y., Petrone, N., Kim, K. S., Roth, M., Hone,}., Kim, P., Pasupathy, 
A. & Pinczuk, A. Large Physisorption Strain in Chemical Vapor Deposition of 
Graphene on Copper Substrates. Nano Letters 12, p. 2408-2413, (2012). 

4 Schiros, T. et al. Connecting Dopant Bond Type with Electronic Structure in 
N-Doped Graphene. Nano Lett 12, p. 4025-4031, (2012). 

5 Wurstbauer, U. et al. Molecular beam growth of graphene nanocrystals on 
dielectric substrates. Carbon 50, p. 4822-4829, (2012). 

6 Goncher, S.J., Zhao, L. Y., Pasupathy, A. N. & Flynn, G. W. Substrate Level 
Control of the Local Doping in Graphene. Nano Letters 13, p. 1386-1392, 
(2013). 

7 Arguello, C.}. et al. Visualizing the charge density wave transition in $2H$- 
${\text{NbSe}}_{2}$ in real space. Physical Review B 89, p. 235115, (2014). 

8 Zhao, L. et al. Local Atomic and Electronic Structure of Boron Chemical 
Doping in Monolayer Graphene. Nano Letters 13, p. 4659-4665, (2013). 

9 Rosenthal, E. P., Andrade, E. F., Arguello, C. J., Fernandes, R. M., Xing, L. Y., 
Wang, X. C., Jin, C. Q., Millis, A. J. & Pasupathy, A. N. Visualization of electron 


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nematicity and unidirectional antiferroic fluctuations at high temperatures in 
NaFeAs. Nature Physics 10, p. 225-232, (2014). 

10 Zabet-Khosousi, A., Zhao, L., Palova, L., Hybertsen, M. S., Reichman, D. R., 
Pasupathy, A. N. & Flynn, G. W. Segregation of Sublattice Domains in 
Nitrogen-Doped Graphene. Journal of the American Chemical Society 136, p. 
1391-1397,(2014). 

11 Okamoto, J.-i., Arguello, C. J., Rosenthal, E. P., Pasupathy, A. N. & Millis, A. J. 
Experimental Evidence for a Bragg Glass Density Wave Phase in a Transition- 
Metal Dichalcogenide. Physical Review Letters 114, p. 026802, (2015). 

12 Arguello, C.}. etal. Quasiparticle Interference, Quasiparticle Interactions, and 
the Origin of the Charge Density Wave in 2H-NbSe2. Physical Review Letters 
114, p. 037001, (2015). 

13 Tsen, A. W. et al. Nature of the Quantum Metal in a crystalline 2D 
superconductor. NatPhys, (2016). 

14 Tsen, A. W. et al. Structure and control of charge density waves in two- 
dimensional lT-TaS2. Proceedings of the National Academy of Sciences 112, p. 
15054-15059,(2015). 

15 Lukashev, P. V., Zhao, L., Paudel, T. R., Schiros, T., Hurley, N., Tsymbal, E. Y., 
Pinczuk, A., Pasupathy, A. & He, R. Modification of the G-phonon mode of 
graphene by nitrogen doping. Applied Physics Letters 108, p. 041907, (2016). 

16 C Gutierrez, L. B., C) Kim, J Park, AN Pasupathy. Klein tunnelling and electron 
trapping in nanometre-scale graphene quantum dots. Nature Physics 
(advanced online publication), (2016). 

17 Christopher Gutierrez, C.-J. K., Lola Brown, Theanne Schiros, Dennis 
Nordlund, Edward B Lochocki, Kyle M Shen, Jiwoong Park, Abhay N 
Pasupathy. Imaging chiral symmetry breaking from Kekule bond order in 
graphene. Nature Physics (advanced online publication), (2016). 

18 Zhou, X., Kang, K., Xie, S., Dadgar, A., Monahan, N. R., Zhu, X. Y., Park, J. & 
Pasupathy, A. N. Atomic-Scale Spectroscopy of Gated Monolayer MoS2. Nano 
Letters 16, p. 3148-3154, (2016). 


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AI-OSR Deliverables Submission Survey 


Response ID:6582 Data 

1 . 

1. Report Type 
Final Report 
Primary Contact E-mail 

Contact email if there is a problem with the report. 

apn2108@columbia.edu 

Primary Contact Phone Number 

Contact phone number if there is a problem with the report 

2128546335 

Organization / Institution name 

Columbia University 

Grant/Contract Title 

The full title of the funded effort. 

MICROSCOPIC ELECTRONIC AND MECHANICAL PROPERTIES OF ULTRA-THIN LAYERED 
MATERIALS 

Grant/Contract Number 

AFOSR assigned control number. It must begin with "FA9550" or "F49620" or "FA2386". 

FA9550-11-1-0010 

Principal Investigator Name 

The full name of the principal investigator on the grant or contract. 

Abhay Pasupathy Narayan 

Program Manager 

The AFOSR Program Manager currently assigned to the award 
Harold Weinstock 
Reporting Period Start Date 

04/01/2011 

Reporting Period End Date 

03/31/2016 

Abstract 

The research goals of this project were to characterize the microscopic electronic and structural properties 
of ultra-thin (few layer) crystalline materials, commonly referred to as 2D materials. The technical approach 
used was to perform atomic-resolution scanning tunneling microscopy along with nanofabrication and 
transport measurements. The materials studied under the grant were graphene and transition metal 
dichalcogenides. In graphene, studies centered on material created by the chemical vapor deposition 
technique. A number of studies were conducted to determine the optimal conditions to grow graphene 
under. The doping of graphene with foreign atoms was investigated extensively. Finally, exotic electronic 
states in graphene that can be induced via substrate and adatom interactions were investigated. In the 
transition-metal dichalcogenides, studies initially focused on the charge density wave materials. It was 
discovered that the charge density wave transition is extremely sensitive to disorder. It was then found that 
the charge density wave is not driven by Fermi surface nesting, but rather by strong electron-phonon 
coupling. Superconductivity in transition metal dichalcogenides was then investigated, and it was found 

that in the limit of thin samples, a new Bose metal state emerges in the presence of a magnetic field. 
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Finally, semiconducting transition metal dichalcogenides grown by metal organic chemical vapor 
deposition were studied and their atomic and electronic quality were measured by scanning tunneling 
microscopy. 

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Archival Publications (published) during reporting period: 

Total of 18, as described in the report. 

2. New discoveries, inventions, or patent disclosures: 

Do you have any discoveries, inventions, or patent disclosures to report for this period? 

No 

Please describe and include any notable dates 

Do you plan to pursue a claim for personal or organizational intellectual property? 

Changes in research objectives (if any): 

None 

Change in AFOSR Program Manager, if any: 

None 

Extensions granted or milestones slipped, if any: 

None 

AFOSR LRIR Number 
LRIR Title 
Reporting Period 
Laboratory Task Manager 
Program Officer 
Research Objectives 
Technical Summary 

Funding Summary by Cost Category (by FY, $K) 



Starting FY 

FY+1 

FY+2 

Salary 




Equipment/Facilities 




Supplies 




Total 





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Report Document 

Report Document - Text Analysis 

Report Document - Text Analysis 

Appendix Documents 

2. Thank You 

E-mail user 

Jul 24, 2016 19:18:35 Success: Email Sent to: apn2108@columbia.edu 


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