nano-bio spectroscopy group - engineering photophenomena in...

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limits size, scalability, and dimensionality. On the other hand, recently demonstrated techniques in the self-assembly of hybrid fl akes of different 2D materials with designer compositions [ 16 ] point the way toward self-assembling larger, 3D structures. Despite the inherent disorder present in a self-assembly pro-cess, these larger 3D structures could provide enhanced func-tionality, now in bulk form, arising from the average response of a large number of hybrid van der Waals fl akes. Like in the case of few-layer, mechanically exfoliated heterostructures, the enhanced functionality could come from combining comple-mentary 2D crystals, as well as from the novel physics at the interfaces of two dissimilar 2D materials. Previously, the liquid exfoliation of 2D nanofl akes into 3D hybrid materials has led to improved mechanical properties of 3D foam, [ 17 ] structural reinforcement of polymers, [ 18 ] and promising thermoelectric properties. [ 18 ] From a practical perspective, large 3D structures also allow the study of optoelectronic responses of van der Waals heterostrucutres at longer wavelengths, where diffrac-tion limited probe spots require larger material sizes. This long wavelength region of the electromagnetic spectrum, such as the terahertz (THz) region, remains important for a variety of sci-entifi c and engineering applications, such as sensing, security,

Engineering Photophenomena in Large, 3D Structures Composed of Self-Assembled van der Waals Heterostructure Flakes

M. Bala Murali Krishna , Michael K. L. Man , Soumya Vinod , Catherine Chin , Takaaki Harada , Jaime Taha-Tijerina , Chandra Sekhar Tiwary , Patrick Nguyen , Patricia Chang , Tharangattu N. Narayanan , Angel Rubio , Pulickel M. Ajayan , Saikat Talapatra , and Keshav M. Dani *

Dr. M. B. M. Krishna, Dr. M. K. L. Man, C. Chin, T. Harada, Prof. K. M. Dani Femtosecond Spectroscopy Unit Okinawa Institute of Science and Technology Graduate University 1919-1 Tancha, Onna-son , Kunigami , Okinawa 904-0495 , Japan E-mail: [email protected] S. Vinod, J. Taha-Tijerina, P. Nguyen, Prof. P. M. Ajayan Department of Materials Science and Nanoengineering Rice University TX 77005 , USA C. S. TiwaryMaterials Engineering Indian Institute of Science Bangalore 560012 , India P. Chang Department of Chemistry Rice University TX 77005 , USA

DOI: 10.1002/adom.201500296

Over the past decade, the discovery of 2D materials with a variety of intrinsic properties [ 1 ] has spawned a new fi eld of condensed matter and materials science research. [ 2–4 ] Newly emerging ideas suggest the assembly of different 2D materials into heterostructures via van der Waals bonding to obtain a near limitless combination of materials capabilities and unexplored functionalities. [ 5,6 ] Toward this aim, few-layer heterostructures of graphene (G), hexagonal boron nitride (hBN), and transi-tion metal dichalcogenides have combined their respective strengths of gate-tunable conductivity, insulating properties and photoresponsivity to demonstrate fl exible, transparent, [ 7 ] atomi-cally scaled [ 8 ] electronics and photovoltaic devices, [ 9 ] as well as rewriteable [ 10 ] or nonvolatile [ 11 ] memory devices. Besides com-bining materials with complementary strengths, van der Waals bonding of two dissimilar materials can result in new physics due to their interface interactions, as predicted for room tem-perature excitonic superfl uidity [ 12 ] and superconductivity, [ 13 ] and as demonstrated in tunable metal–insulator transitions [ 14 ] and bandgap engineering. [ 15 ]

These initial demonstrations of van der Waals heterostruc-tures have been restricted to few layers, largely because of the requirements of layer-by-layer mechanical assembly, which

Prof. T. N. Narayanan TIFR Center for Interdisciplinary Sciences (TCIS) Tata Institute of Fundamental Research Hyderabad 500075 , India Prof. A. Rubio Max Planck Institute for the Structure and Dynamics of Matter Hamburg , Germany Prof. A. Rubio Nano-Bio Spectroscopy Group and European Theoretical Spectroscopy Facility (ETSF) Universidad del País Vasco CFM CSIC-UPV/EHU-MPC and DIPC 20018 San Sebastián , Spain Prof. S. Talapatra Femtosecond Spectroscopy Unit Okinawa Institute of Science and Technology Graduate University 1919-1 Tancha, Onna-son , Kunigami , Okinawa 904-0495 , Japan Prof. S. Talapatra Department of Physics Southern Illinois University Carbondale , IL 62901-4401 , USA

Adv. Optical Mater. 2015, DOI: 10.1002/adom.201500296

www.MaterialsViews.comwww.advopticalmat.de

http://doi.wiley.com/10.1002/adom.201500296

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ON and optoelectronic devices. [ 19,20 ] The possibility to obtain fun-

damentally new, engineer-able optoelectronic properties in the THz by appropriately choosing the materials composition of the hybrid 3D structures provides tremendous opportunities for such THz technologies, [ 20 ] as well as in the search for new materials to overcome fundamental limitations in metamaterial applications. [ 21 ]

In this study, we self-assemble large, micrometer-scale, robust 3D structures formed from interlinked, overlapping van der Waals heterostructure fl akes made of atomically thin layers of hBN and G. Using optical pump terahertz probe (OPTP) spectroscopy, we see the emergence of a dissipationless, purely capacitive photoresponse in the hybrid solids, fundamentally distinct from the insulating and conductive response of the parent 2D materials. This response, arising from the average over a large number of heterostructure fl akes in the solid, lasts for several picoseconds and can be explained by charge separa-tion and dipole formation at the hBN/G interfaces. [ 12,16,22 ] We engineer this photoresponse over an unprecedented range—from insulating to capacitive to conductive simply by varying the hBN:G ratio in the van der Waals fl akes during the self-assembly process.

We fabricate the 3D hybrid structures by extending on pre-vious work [ 16 ] demonstrating the self-assembly of mono- to few-layer fl akes of hBN and G in varying ratios to form atomically thin van der Waals heterostructure fl akes ( Figure 1 A). A typical fl ake, as in ref. [ 16 ] , is made of few hundred nanometer diam-eter sheets of hBN, atomically integrated to few micrometer diameter sheets of G (see the Experimental Section). A detailed analysis of the fl akes, including their materials composition,

structure, and properties, has been previously reported. [ 16 ] In this work, we then spray coat the heterostructure fl akes onto a substrate to form an interlinked, overlapping network, which can be built up in the vertical direction by three orders of magnitude to form thick micrometer-scale hybrid fi lms. To test the integrity of the spray-coated fi lms as solid 3D struc-tures, we study the possibility of patterning and manipulating them using standard nanofabrication tools. We observe that the hybrid structures demonstrate greater than expected rigidity and robustness that allows them to be cut into fairly complex 3D structures, such as the Okinawa institute of science and technology (OIST) logo (Figure 1 B), or micropillars out of the (Figure 1 C) G, (D) hBN/G 1:3 and (E) hBN solids. The patterned hybrid structures can also be cleaved and lifted (Figure 1 F) by a micromanipulator tip fused to a thin platinum layer depos-ited on top. This demonstration of robust micrometer-sized 3D structures formed by interlinked, overlapping van der Waals heterostructure fl akes of different atomically thin crystals natu-rally opens up their potential for a variety of applications. To this end, further understanding of their material properties, such as their optoelectronic and photoconductive responses, is important and explored next.

In the past, OPTP spectroscopy [ 23 ] has proved to be a pow-erful and insightful tool into optoelectronic phenomena and the dynamics of photocarriers, such as the formation of exci-tons in TiO 2 nanotubes, [ 24 ] photoconductivity dynamics in gated graphene devices [ 25–27 ] and the formation of electronic correla-tions in metals and semiconductors. [ 28 ] To study the photocon-ductivity dynamics, we excite the few micrometer thick hybrid fi lms on quartz substrates with 800 nm femtosecond pump



Figure 1. 3D hybrid structures made of van der Waals heterostructure fl akes. A) TEM image of a single heterostructure fl ake self-assembled from atomically thin hBN and G layers. The smaller inserts show the energy-fi ltered TEM composition mapping of carbon, boron, and nitrogen content, demonstrating the van der Waals bonding between the hBN and G layers within a fl ake. These heterostructure fl akes are spray coated onto a substrate, overlapping and interlinking to form micrometer-sized, robust hybrid fi lms that can be patterned into 3D structures. B) A micrometer-scale 3D structure of the OIST logo patterned from the spray-coated hBN/G 1:3 hybrid fi lm using a focused ion beam; C–E) micropillars patterned from the graphene, hBN/G 1:3, and hBN fi lms, respectively; and F) a cleaved and lifted hBN/G 1:3 hybrid structure. These demonstrate a surprising and unexpected robustness of the hybrid structures allowing their patterning and manipulation as 3D objects.

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pulses and probe with time-delayed THz pulses (see the Experi-mental Section).

Figure 2 shows the change in transmission of the peak of the THz probe at different time delays after the pump pulse exci-tation (zero delay) for hybrid fi lms of different hBN/G ratios. For the pure hBN structure, one cannot excite carriers above the bandgap with 800 nm (1.5 eV) photons, and thus one sees no change in the transmitted THz pulse (Δ T / T = 0), demon-strating insulating behavior. At the other extreme, for the purely G phase, we see an instantaneous drop in THz transmission (Δ T / T < 0) on photoexcitation—caused by the screening of the transmitted THz pulse by the free photocarriers, demonstrating the expected picosecond timescale, semiconducting response of the purely G solid. [ 29–32 ] On the other hand, a strikingly different photoresponse emerges in hybrid fi lms containing comparable proportions of G and hBN. For the hBN/G 1:1 and 1:3 hybrid fi lms, after about 1 ps, we see the change in THz transmission becomes positive (Δ T / T > 0) and lasts for several picoseconds. This is in stark contrast to the photoconductive (Δ T / T < 0) and insulating (Δ T / T = 0) behavior of the G and hBN parent struc-tures, respectively. We note that the observed response is seen in multiple measurements for multiple hybrid fi lms and is not limited by the inherent statistical variation in self-assembled samples.

This measure of the change in transmission of the peak of the THz probe on photoexcitation provides an initial, but lim-ited understanding of the carrier dynamics. A complete picture of the photoconductivity of the carriers requires a measurement of the full frequency dependent real and imaginary parts of the

photoconductivity of the carriers. To this end, we next measure the frequency-dependent complex photoconductivity response (Δ σ ) at different pump–probe delays [ 23,31 ] (see the Experimental Section). Figure 3 shows the real (Δ σ Real ) and imaginary (Δ σ Img ) com-ponents, for the (a) hBN/G 1:3 hybrid fi lm and for the (b) purely G solid. At zero time delay, where the peak THz Δ T / T is negative, both solids show similar complex photo-conductivity—a positive value of Δ σ Real and a nonzero, negative value for Δ σ Img , cor-responding to mobile carriers in a network of 2D nanofl akes. [ 31 ] In the purely G solid, the photoconductivity response at later time delays continues to be qualitatively similar to this zero-delay response.

On the other hand, the complex photo-conductivity response of hBN/G 1:3 at later time delays, where the change in transmis-sion of the THz peak Δ T / T > 0, demon-strates a striking departure from the purely G structure. In Figure 3 A(iii),(v), Δ σ Real = 0, indicating dissipationless photoconduc-tivity in the measured frequency range. The Δ σ Img part (Figure 3 A(iv),(vi)) exhibits a negative linear dependence on frequency corresponding to a purely photocapacitive response. [ 31 ] As the density of photocarriers decays over time, this capacitance (given

by the negative slope) decays to zero, demonstrating the tun-ability of the strength of the photocapacitance with carrier density. This dissipationless, purely capacitive photo response, verifi ed in multiple hBN/G 1:3 fi lms (Figure S2, Supporting Information), is distinctly different from the insulating and photoconductive responses of the parent 2D materials of the heterostructures. We understand the repeatability and repro-ducibility of this photoresponse across multiple samples to be due an averaging effect over the relatively large measurement spot size (≈few mm), in comparison to the potential disorder introduced in the sample during the self-assembly process of micrometer-sized hybrid fl akes. In general, we expect that the photoresponse of the material will be inhomogeneous on the micrometer scale due to the size of individual heterostructure fl akes and the disorder inherent in their individual formation during self-assembly. On the other hand, we expect the photore-sponse to be homogeneous on the millimeter scale, whereby the average response of a large number of fl akes is measured.

We also note that the OPTP response of the hBN/G hetero-structure shows a fundamentally distinct response not just from the parent materials but also from recent OPTP measurements on doped or gated single layer graphene samples. [ 27,33 ] Despite the similar positive to negative transition in the change in trans-mission of the peak of the THz probe on photoexcitation, a very different picture emerges when comparing the above frequency dependent real and imaginary parts of the photoconductivity of the carriers. In the case of gated graphene, one measures a nonzero Δσ Real that is positive or negative—determined by the Fermi level of the gated graphene, and corresponding to the



Figure 2. Tuning instantaneous THz photoresponse by varying composition of the hetero-structure fl akes within 3D hybrid fi lms. The change in transmission of THz peak (Δ T / T ) due to photoexcitation versus pump–probe delay for different heterostructure compositions shows an insulating (Δ T / T = 0) response for the purely hBN structure, to a distinctly different photore-sponse (explored in more detail in Figure 3 ) for hBN/G 1:1 and 1:3 heterostructure (Δ T / T > 0 after 1 ps) and evolving to a photoconductive response for the purely G structure (Δ T / T < 0). Inset shows the basic concept of an Optical Pump THz Probe experiment with a cartoon sche-matic representing the self-assembled hBN and G layers within the heterostructure.


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reported semiconducting to metallic transition. In the case of the hBN/G hybrid fi lms, the dissipationless, photocapacitive response (Δ σ Real = 0 and Δ σ Img a negative linear function of frequency) is a fundamentally different optoelectronic response and cannot be attributed simply to metallic or semiconducting behavior. With this, we turn our attention to understanding the underlying physics behind the measured dissipation-less, pho-tocapacitive frequency dependent response.

To explain the time resolved photocarrier response of the hybrid fi lms, we assume that the photocarriers generated in the graphene layers separate at the hBN/G interfaces to form dipoles [ 12,16,22 ] within a picosecond. The photoconductivity of the hybrid fi lm is then given by the rapidly decaying free-carriers (Drude–Smith response) and the longer-lived dipoles (Drude–Lorentz response). [ 31 ] (See the Experimental Section for further details of the model.) Figure 3 A(i)–(vi) (red curves) shows the calculated frequency-dependent Δ σ Real and Δ σ Img at different time delays based on this model. Our simulations fi t the data very well and demonstrate that a picosecond after excitation in the hBN/G 1:3 hybrid fi lm, free carriers do not contribute to the measured photoconductivity (Δ σ Real = 0). The purely capacitive response comes from the dipoles at the large number of hBN/G interfaces in the hybrid fi lms.

In summary, our work demonstrates the emergence of new, engineerable optical phenomena in large, robust 3D struc-tures formed from van der Waals heterostructure fl akes. This observed phenomenon is distinct from the photophenomena seen in the hBN and G parent materials as well as recently demonstrated gated graphene devices. The ability to engineer the photoresponse of our hybrid fi lms over the broad range from insulating to capacitive to conductive, simply by varying the ratio of hBN/G in the van der Waals heterostructure fl akes, creates opportunities in tunable THz devices and versatile

optoelectronic materials. The fabrication of a 3D bulk mate-rial displaying dissipationless photoconductivity could also fi nd utility in metamaterial structures in order to mitigate losses. Further developments in the self-assembly process, for example, fi ltration of the layers to restrict the lateral size distribution, or control over the stacking sequence, will lead to enhancement and control of optoelectronic properties with direct benefi t to the above mentioned applications. From a fundamental per-spective, further developments in broader bandwidths for the THz probe, will provide direct access to the dipole resonance formed between the hBN/G interfaces. This in turn will provide new tools in the investigation of the physics of these exciton-like formations. More broadly, our work extends to the fabrica-tion and investigation of other 3D structures made of van der Waals heterostructure fl akes self-assembled from the diverse library of available 2D materials. In general, such hybrid mate-rials with designer compositions and the potential for complex 3D architectures pave the way to a rich combinatorics of mate-rials synthesis, functionality, and device applications.

Experimental Section Sample Fabrication : Mono- to few-layer fl akes of hBN and G

are fi rst independently exfoliated from their bulk powders using dimethylformamide. The exfoliated hBN and G fl akes are fi ltered and redispersed separately in tetrahydrofuran (THF) and then mixed together in specifi c volume ratios. Via van der Waals bonding, this leads to the formation of hBN/G hybrid fl akes. We spray coat these hybrid fl akes onto a quartz or silicon substrate and allow for the solvent (THF) to evaporate. The procedure is repeated to form fi lms of a few micrometer thickness of the hybrid fi lm with the specifi ed composition on 1 square inch silicon or quartz substrates. For the purposes of this study, we made purely hBN and purely G solids, along with hybrid fi lms



Figure 3. Frequency dependent THz photoconductivity of the hybrid fi lms at different time delays. A) hBN/G 1:3 hybrid fi lm: at zero delay, (i) Δ σ Real and (ii) Δ σ Img show the dominant Drude–Smith response. By 4 ps, corresponding to the positive change in transmission of the THz peak positive (upper panel and Figure 2 for the hBN/G 1:3 hybrid fi lm), the photoconductivity evolves into (iii) Δ σ Real = 0, i.e., dissipationless conductivity over the meas-ured THz range and (iv) Δ σ Img showing a negative linear relationship in frequency, i.e., a photocapacitive response. At 8 ps, (v) Δ σ Real and (vi) Δ σ Img exhibit the same dissipationless, purely capacitive photoresponse, but with a decreased capacitance. Red curves in panels (i)–(vi) are simulations of the model described by a free carrier response evolving into the response of bound dipoles (see the Experimental Section). In contrast, the B) purely G structure shows a negative change in THz transmission and a dissipative (Δ σ Real ≠ 0) frequency-dependent THz photoresponse at all time delays. This is primarily a free-carrier response as also seen at zero delay in the hBN/G 1:3 hybrid fi lm.

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with hBN/G ratios of 1:1, 1:3, and 1:5. The structures on silicon were patterned using a focused ion beam, while the large area samples on quartz were used for the photoconductivity measurements.

Materials Characterization : To confi rm the atomic-scale integration of hBN and G in our heterostructure samples, we disperse the hybrid fl akes in ethanol solution by sonication for 1 h and drop cast them onto a transmission electron microscopy (TEM) grid (Cu mesh with holey carbon coating) grid. The fl akes are then imaged with a JEOL2100 TEM with a fi eld emission gun operating at 80–200 kV. analysis is performed using a GATAN EELS attachment to the microscope. Image analysis is performed using the GATAN Digital Microanalysis software. Low-magnifi cation TEM images (Figure S1a, Supporting Information) show small few-hundred nanometer sheets of BN attached to larger few micrometer sheets of G. High resolution TEM images and intensity profi les (Figure S1b–e, Supporting Information) show that the hBN and G sheets are typically mono- to few-layers thick and exhibit atomic-scale integration. The measurements were repeated with multiple samples and no separate hBN fl akes or G fl akes were observed in our heterostructure samples.

Experimental Technique : The OPTP measurements are performed on the samples on quartz substrates using a source laser with 1 kHz repetition rate, 5 mJ pulse energies, 800 nm center wavelength, and 70 fs pulse duration. The laser pulses are split into a pump pulse and a time-delayed probe pulse. The attenuated pump pulse of energy 40 μJ excites ≈10 17 cm −3 photocarriers in the sample within a spot size of 5 mm diameter. The probe pulse generates a subpicosecond, single-cycle THz pulse via optical rectifi cation in a 1 mm ZnTe crystal to probe the photocarrier dynamics in the sample. The transmitted THz probe is then measured via electrooptic sampling in a second ZnTe crystal with a delay-adjustable gating pulse split from the probe. To measure the pump-induced change in transmission of the THz peak (Δ T / T ) versus pump–probe delay (Δ t ) (Figure 2 ), the delay of the gating pulse is set to measure the peak of the THz probe in the time domain, and the pump delay is scanned. To measure the frequency-dependent complex photoconductivity (Figure 3 ), the pump pulse is parked at the chosen pump–probe delay (Δ t = 0, 4, or 8 ps), and the gating pulse is scanned to obtain the pump-induced change in transmission of the entire THz waveform in time domain. This is then Fourier transformed to the frequency domain and using the thin fi lm approximation the complex photoconductivity of the hybrid fi lm is given by [ 33 ]

σΔ = − +⎛

⎝⎜⎞⎠⎟

Δ1 1s

0Ln

zT

T (1)

where L = 5 μm is the thickness of the hybrid fi lm, n s = 2.1 is the refractive index of the substrate and Z 0 = 377 Ω is the impedance of free space. We confi rm the validity of the thin fi lm approximation by verifying that conditions n h ωL / c = 1 and Δ T = T are satisfi ed. [ 34 ] Here, n h , the refractive index of the photoexcited hybrid fi lm, is estimated to be much less than 10 from THz time domain measurements and Δ T / T is measured to be on the order of 10 −3 .

Modeling Complex Photoconductivity : The time-evolving photoconductivity (Δ σ ) of the hybrid fi lm is explained by the combined contribution of decaying free carriers and long-lived dipoles. The free carrier response is modeled by the standard Drude–Smith equation for the case of a network of nanoparticles, [ 31 ] given by

σ ωτ ωτ( )= − + −

11 i

11 iFree A c

(2)

where c is the backscattering parameter, τ is the scattering time, and A is proportional to the density of free carriers with a 0.6 ps decay time. The contribution of dipoles is given by the Drude–Lorentz equation:

σ

τ ω ω ω( )=− −

⎛

⎝⎜

⎞

⎠⎟

11 i /

Dipole02

L

(3)

where ω is the resonance dipole frequency and L is proportional to the dipole density with a decay time of 5 ps. Thus the combined time-evolving complex photoconductivity of the hybrid fi lm is given by

σ ωτ ωτ

τ ω ω ω

( )( )

( )Δ Δ = − + −

+− −

⎛

⎝⎜

⎞

⎠⎟

−Δ

−Δ

e 11 i

11 i

e 11 i /

0/0.6 ps

0/5 ps

02

t A c

L

t

t

(4)

The values of the parameters in the above equation used to fi t the data in Figure 3 A for all time delays are given by A 0 = 0.1 S cm −1 , c = −0.75, L 0 = 5 S cm −1 , ω 0 = 12 THz, and τ = 0.65 ps.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements K.M.D. and S.T. acknowledge the support provided by Japan Society for the Promotion of Science (JSPS) through a JSPS long term invitation fellowship. S.V. acknowledges funding support from EAGER—NSF ECCS-1327093. J.T.-T. acknowledges fi nancial support from CONACyT (213780). A.R. acknowledges fi nancial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG-267374), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13), and EC FP7 project CRONOS (Grant No. 280879-2).

Received: May 31, 2015 Revised: July 8, 2015

Published online:

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