Physics Degree Level: Bachelor Abstract ID# 692
Undergraduate Category: Physics Degree Level: Bachelor Abstract ID# 692
Synthesis and Characterizations of Two-Dimensional Heterojunctions Matthew DeCapua, Fangze Liu and Swastik Kar Department of Physics, Northeastern University
Abstract Inspired by the superior properties of graphene, an atomically thin, twodimensional (2D) allotrope of carbon, that result from its two-dimensional electronic structure, a number of other 2D materials have been investigated in recent times. One such material is molybdenum disulfide (MoS2) which exhibits a transition from indirect band gap for bulk material to direct bandgap for a single layer, making it a promising material for nano optoelectronic devices. An area of particular interest is the properties of MoS2-graphene 2D heterojunctions. Here we probe the optoelectronic properties, including photocurrent, power conversion efficiency, and wavelength dependence of MoS2-graphene heterojunctions grown by chemical vapor deposition, in an attempt to demonstrate its efficacy in producing useful materials on large scales. Characterization methods include Raman and photoluminescence spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM). We also examine the Quantum Carrier Reinvestment (QCR) phenomenon in MoS2graphene junctions, considering its dependence on electrical voltage, devices size, and recombination lifetimes. QCR photocurrent spectroscopy are investigated to perform an in-depth investigation of the timescale of electronic transitions at these junctions.
Methods • • • • •
Graphene was grown by CVD on copper foil. Transfer graphene from Cu foil to silicon substrate assisted by PMMA spin coating. MoS2 was grown by CVD on transferred graphene. MoS2/graphene heterostructure was patterned by oxygen plasma. Gold electrodes were constructed by electron beam lithography and electron beam deposition.
Microscope objective
Au/Ti contact
MoS2/Graphene
Sourcemeter VDC
Monochromator
Photograph of two typical MoS2/graphene QCR photodetectors with the external circuit diagram. Scale bar is 100 µm. Transfer PMMA/graphene film to silicon substrate
Current (mA)
Results
0.635
(b)
Introduction Graphene consists of single layer carbon atoms organized in a hexagonal lattice, while MoS2 has a similar structure but with three layers of atoms. A few reports and our preliminary results show that MoS2 can be epitaxially grown on graphene despite the large lattice mismatch by using chemical vapor deposition (CVD), a production method that is easily scalable to larger areas. This direct synthesis will produce defect-free and atomically clean MoS2-graphene interface which is an ideal system to study the lightmatter interactions in atomically thin films. These heterojunctions have strong light-matter interactions, leading to opportunities for new design in photodetection, photovoltaics, optoelectronic switches or memory devices.
Light on Light Off
15
Photocurrent (μA)
Etch away copper with nitric acid, leaving PMMA/graphene film floating
Graphene grown by CVD Spin-coat PMMA on on Cu foil graphene/Cu foil
(a)
0.63 0.625 0.62 0.615
(a) SEM image of MoS2 on graphene. Yellow dotted line indicates graphene crystal boundary, red lines show favored MoS2 crystal orientation. Scale bar is 2 µm (b) AFM image of MoS2/graphene hybrid structure. The step height of 0.7 nm equals to single layer MoS2.
C S Mo
(b)
4
3
(c)
1 E2g
G’
2
A1g
1
G D
0 1.6
1.8
2.0
2.2
Photon energy (eV)
Atomic schematic of MoS2 grown on graphene with preferred orientation. (a) View along the c axis ([001] plane). (b) View along the a axis ([100] plane). (c) 3D view reveals the MoS2 lattice structure.
2.4
2.6
360
370
380
390
400
Raman shift (cm-1)
410
420
430
10 20 Time (seconds)
30
600 620 640 660 680 700 Incident Wavelength (nm)
Conclusion • • •
Epitaxial growth of single and few layers MoS2 on single layer graphene by all-CVD method. Strong photoresponse was observed in MoS2/graphene heterostructure. The wavelength dependence of photocurrent and the high responsivity confirm that photoexcited carriers were generated from MoS2, not from graphene.
References
Raman signal (a.u.)
(c)
Photoluminescence signal (a.u.)
(a)
5
(a) Photocurrent response under 600nm (blue) and 700nm (red). (b) Photocurrent as a function of incident wavelength with power level of 1 µW. The responsivity is over 10 A/W at 600 nm, much higher than the intrinsic photoresponse in graphene. The photocurrent vanished around 700 nm (~1.8 eV) which is equal to the direct bandgap of single layer MoS2, suggesting the photoexcited carriers were generated in MoS2.
0.7 nm
(b)
10
0 0
Raman signal (a.u.)
(a)
Au/Ti contact
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
Raman shift (cm-1)
(a) Photoluminescence spectrum of MoS2 on graphene. The Raman peaks of graphene and MoS2 are also shown. (b) Raman spectrum of MoS2 shows two prime peaks at 382 cm-1 and 401 cm-1, suggesting monolayer MoS2. (c) Raman spectrum of graphene shows a small D peak and a G’ to G ration ~2, suggesting high quality monolayer graphene.
1. Geim, A. K. et al. The rise of graphene. Nature Materials 2007, 6, 183–191 2. Britnell, L. et al. Strong Light-Matter Interactions in Heterostructures of Atomically Thin Films Science 2013, 340, 1311-1314 3. Shi, Y. et al. van der Waals Epitaxy of MoS2 Layers Using Graphene As Growth Templates. Nano Letters, 2012, 12 (6), 2784–2791 4. Li, X. et al. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science 2009, 324, 1312-1314 5. Liu, F. et al. Quantum Carrier Reinvestment-Induced Ultrahigh and Broadband Photocurrent Responses in Graphene-Silicon Junctions. ACS Nano 2014, 8, 10270-10279
Acknowledgement This project was partially supported by NSF ECCS 1351424. Matthew DeCapua gratefully acknowledges a co-op fellowship provided by Northeastern University as well as a Physics Department Lawrence Undergraduate Research Award.
Synthesis and Characterizations of Two-Dimensional Heterojunctions Matthew DeCapua, Fangze Liu and Swastik Kar Department of Physics, Northeastern University
Abstract Inspired by the superior properties of graphene, an atomically thin, twodimensional (2D) allotrope of carbon, that result from its two-dimensional electronic structure, a number of other 2D materials have been investigated in recent times. One such material is molybdenum disulfide (MoS2) which exhibits a transition from indirect band gap for bulk material to direct bandgap for a single layer, making it a promising material for nano optoelectronic devices. An area of particular interest is the properties of MoS2-graphene 2D heterojunctions. Here we probe the optoelectronic properties, including photocurrent, power conversion efficiency, and wavelength dependence of MoS2-graphene heterojunctions grown by chemical vapor deposition, in an attempt to demonstrate its efficacy in producing useful materials on large scales. Characterization methods include Raman and photoluminescence spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM). We also examine the Quantum Carrier Reinvestment (QCR) phenomenon in MoS2graphene junctions, considering its dependence on electrical voltage, devices size, and recombination lifetimes. QCR photocurrent spectroscopy are investigated to perform an in-depth investigation of the timescale of electronic transitions at these junctions.
Methods • • • • •
Graphene was grown by CVD on copper foil. Transfer graphene from Cu foil to silicon substrate assisted by PMMA spin coating. MoS2 was grown by CVD on transferred graphene. MoS2/graphene heterostructure was patterned by oxygen plasma. Gold electrodes were constructed by electron beam lithography and electron beam deposition.
Microscope objective
Au/Ti contact
MoS2/Graphene
Sourcemeter VDC
Monochromator
Photograph of two typical MoS2/graphene QCR photodetectors with the external circuit diagram. Scale bar is 100 µm. Transfer PMMA/graphene film to silicon substrate
Current (mA)
Results
0.635
(b)
Introduction Graphene consists of single layer carbon atoms organized in a hexagonal lattice, while MoS2 has a similar structure but with three layers of atoms. A few reports and our preliminary results show that MoS2 can be epitaxially grown on graphene despite the large lattice mismatch by using chemical vapor deposition (CVD), a production method that is easily scalable to larger areas. This direct synthesis will produce defect-free and atomically clean MoS2-graphene interface which is an ideal system to study the lightmatter interactions in atomically thin films. These heterojunctions have strong light-matter interactions, leading to opportunities for new design in photodetection, photovoltaics, optoelectronic switches or memory devices.
Light on Light Off
15
Photocurrent (μA)
Etch away copper with nitric acid, leaving PMMA/graphene film floating
Graphene grown by CVD Spin-coat PMMA on on Cu foil graphene/Cu foil
(a)
0.63 0.625 0.62 0.615
(a) SEM image of MoS2 on graphene. Yellow dotted line indicates graphene crystal boundary, red lines show favored MoS2 crystal orientation. Scale bar is 2 µm (b) AFM image of MoS2/graphene hybrid structure. The step height of 0.7 nm equals to single layer MoS2.
C S Mo
(b)
4
3
(c)
1 E2g
G’
2
A1g
1
G D
0 1.6
1.8
2.0
2.2
Photon energy (eV)
Atomic schematic of MoS2 grown on graphene with preferred orientation. (a) View along the c axis ([001] plane). (b) View along the a axis ([100] plane). (c) 3D view reveals the MoS2 lattice structure.
2.4
2.6
360
370
380
390
400
Raman shift (cm-1)
410
420
430
10 20 Time (seconds)
30
600 620 640 660 680 700 Incident Wavelength (nm)
Conclusion • • •
Epitaxial growth of single and few layers MoS2 on single layer graphene by all-CVD method. Strong photoresponse was observed in MoS2/graphene heterostructure. The wavelength dependence of photocurrent and the high responsivity confirm that photoexcited carriers were generated from MoS2, not from graphene.
References
Raman signal (a.u.)
(c)
Photoluminescence signal (a.u.)
(a)
5
(a) Photocurrent response under 600nm (blue) and 700nm (red). (b) Photocurrent as a function of incident wavelength with power level of 1 µW. The responsivity is over 10 A/W at 600 nm, much higher than the intrinsic photoresponse in graphene. The photocurrent vanished around 700 nm (~1.8 eV) which is equal to the direct bandgap of single layer MoS2, suggesting the photoexcited carriers were generated in MoS2.
0.7 nm
(b)
10
0 0
Raman signal (a.u.)
(a)
Au/Ti contact
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
Raman shift (cm-1)
(a) Photoluminescence spectrum of MoS2 on graphene. The Raman peaks of graphene and MoS2 are also shown. (b) Raman spectrum of MoS2 shows two prime peaks at 382 cm-1 and 401 cm-1, suggesting monolayer MoS2. (c) Raman spectrum of graphene shows a small D peak and a G’ to G ration ~2, suggesting high quality monolayer graphene.
1. Geim, A. K. et al. The rise of graphene. Nature Materials 2007, 6, 183–191 2. Britnell, L. et al. Strong Light-Matter Interactions in Heterostructures of Atomically Thin Films Science 2013, 340, 1311-1314 3. Shi, Y. et al. van der Waals Epitaxy of MoS2 Layers Using Graphene As Growth Templates. Nano Letters, 2012, 12 (6), 2784–2791 4. Li, X. et al. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science 2009, 324, 1312-1314 5. Liu, F. et al. Quantum Carrier Reinvestment-Induced Ultrahigh and Broadband Photocurrent Responses in Graphene-Silicon Junctions. ACS Nano 2014, 8, 10270-10279
Acknowledgement This project was partially supported by NSF ECCS 1351424. Matthew DeCapua gratefully acknowledges a co-op fellowship provided by Northeastern University as well as a Physics Department Lawrence Undergraduate Research Award.
