Supporting Information Highly Scalable, Atomically Thin WSe2 grown ...
Supporting Information
Highly Scalable, Atomically Thin WSe2 grown via Metal-Organic Chemical Vapor Deposition Sarah M. Eichfeld, 1,2 Lorraine Hossain,1,2 Yu-Chuan Lin,1,2 Aleksander F. Piasecki,1,2 Benjamin Kupp,1,2 A. Glen Birdwell,4 Robert A. Burke,4 Ning Lu,3 Xin Peng,3 Jie Li,2,5 Angelica Azcatl,3 Stephen McDonnell,3 Robert M. Wallace,3 Theresa S. Mayer,2,5 Moon J. Kim,3 Joan M. Redwing,1,2 and Joshua A. Robinson1,2* 1. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, United States 2. Center for Two-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, United States 3. Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States 4. US Army Research Laboratory, Adelphi, Maryland 20783, United States 5. Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802, United States *E-mail: [email protected]
The following pages include supporting figures and experimental data. 1. Growth of WSe2: WSe2 is synthesized via MOCVD in a cold wall vertical reactor system. The gas manifold has two bubbler stations equipped with W(CO)6 and (CH3)2Se allowing for
individual control of each precursor. The bubbler pressure is held at 700 and 760 Torr respectively, and bubblers are held at room temperature. The MOCVD system is equipped with both H2 and N2 that can be used as the carrier gas.
Figure S1 a) Schematic of MOCVD process allowing for precise precursor control in a vertical cold wall system for the investigation of the synthesis conditions b) AFM of WSe2 on sapphire after growth showing monolayer WSe2 can be achieved c) Raman spectra displaying the E12g/A1g peak around 250 cm-1 indicating high quality WSe2. d) PL of monolayer WSe2 on sapphire. e) Raman map of A1g intensity showing additional layers on sapphire nucleate from edge of domains as opposed to the center when synthesized on sapphire substrates.
2. XPS of WSe2 XPS of the as grown WSe2 samples were carried out using a monochromated Al Kα source (hv = 1486.7 eV), equipped with a 7 channel analyzer. The XPS spectra were acquired at a pass energy of 15 eV and a take-off angle of 45°. An Omicron CN10 electron flood source was employed for charge neutralization during XPS scanning. XPS peak deconvolution was performed using the spectral analysis software analyzer where Voigt line shapes and an active Shirley background were used for peak fitting.1 The Se/W ratio and the WO3/WSe2 ratio were then determined (Fig. S2, T1) for synthesis using 100% H2 as the carrier gas compared to a 1:3 H2:N2 ratio. The peak positions for Se 3d and W 4f were affected by sample charging due to the insulating properties of the sapphire. This was corrected using a thin layer of Au deposited on the sample surface in addition to a charge neutralizer.
Figure S2: XPS binding energy for W 4f and Se 3d peaks comparing 100% H2 as the carrier gas to a 1:3 H2:N2 ratio. The characteristic tungsten region shows both W 4f and W 5p3/2 with the W5p3/2 appearing at 39.4 eV. Therefore, the W 5p3/2 peak overlaps with the WO3 peaks (deconvolution of such peaks are shown in the figure). The Se/W ratio was calculated using the integrated intensity of the Se 3d and W 4f peaks. Table T1 below shows a Se/W and WO3/WSe2 ratiosfor samples grown using 100% H2 versus 1:3 H2:N2 ratio. Growth with 100% H2 results in a Se/W ratio of 2, while growth with a 1:3 H2:N2 ratio results in a Se/W ratio of 1.8 which is Se deficient. The WO3/WSe2 ratio is higher for the 100% H2 however this sample was grown prior to the sample synthesized in a H2/N2 ambient. WSe2 can be highly sensitive to the air, so this could account for the slightly higher ratio of WO3/WSe2. Table T1: Se/W ratio and WO3/WSe2 ratio for samples grown using 100 % H2 as compared to samples grown using a 1:3 H2:N2 ratio. Samples
Se/W Ratio
WO3/WSe2 Ratio
100 % H2
2.0
0.25
1:3 H2:N2
1.8
0.09
The deconvolution of C incorporation in these samples is difficult due to overlap of the
carbon 1s peak with the Se Auger feature, so it is necessary to examine Raman spectra of these two samples to further determine the sample quality and if there is any carbon present in the Raman spectra (Fig S3). The PL for the sample grown using a H2/N2 ambient was quenched as opposed to strong PL for the sample grown in H2. The domain size and layer thickness was similar for both samples.
Figure S3: The impact of H2 on the growth of WSe2. (a) Raman spectra comparing 100% H2 versus 1:3 H2:N2 as the carrier gas for synthesis of WSe2. A H2:N2 mix for the carrier gas shows the carbon impurity as seen by D and G peaks. The PL is also quenched under the presence of carbon in the WSe2. The WSe2 material quality can be highly sensitive to the synthesis process including H2 concentration and precursor quality. The purity of the dimethylselenium ((CH3)2Se) was investigated, and two different precursor vendors including SAFC HiTech (99.99% purity) and STREM Chemical (99.0% purity) were utilized. The use of a lower purity source lead to carbon incorporation as shown from the Raman and PL in Figure S4.
Figure S4: The impact of the impurity in Se precursor on the WSe2 monolayer under the same growth conditions. The red line indicating a Se source purity of 99.0% and the black curve indicating a purity of 99.99%: (a) Raman spectra indicating that the Se precursor with higher impurity yielded carbon impurity incorporation in the WSe2 layers and impacts the electronic structures as evidenced by the quenched PL (b) on samples grown with lower Se precursor purity.
3. Impact of Substrate: The synthesis of WSe2 led to large differences in the final product based on choice of substrate. Initial evidence from both Raman and PL suggests there is some interaction at the WSe2/sapphire interface. Cross-sectional TEM image (Fig. S5) reveals there is some disorder present directly at the interface between the WSe2 and sapphire substrate.
Figure S5: TEM cross-section showing some disorder at WSe2/sapphire interface suggesting possible interaction between WSe2 and the sapphire substrate.
Shifts in the PL peak position for WSe2 on CVD graphene suggests there is some strain in the layer. This was further investigated by examining shifts in the Raman 2D and G peaks in Fig. S6a. The data are vector decomposed (Fig. S6b) to correlate peak shifting to tensile and compressive strain (eT and eC, respectively), Fermi velocity reduction (eFVR), and hole doping (eH), using methods by Ahn et al.2
Figure S6: The presence of strain in WSe2 on CVD graphene. a.) Raman spectra of CVD graphene compared to WSe2 on CVD graphene normalized to the SiO2 at 520 cm-1 showing significant G and 2D peak shifts to higher frequency. b.) Plot of Raman 2D frequency vs. G peak frequency for CVD graphene on SiO2 (black) compared to annealed CVD graphene/SiO2 (blue) and WSe2 growth on CVD Graphene/SiO2 (red). The WSe2 growth resulted in 0.4% compressive strain, while the same growth condition without W and Se sources introduced resulted in 0.2 % compressive strain comparing to a (freestanding graphene sample). This indicates that the WSe2 deposited on CVDGr contributes additional 0.2% strain in addition to the strain from the thermal effects. The strain in monolayer CVDGr is calculated according to Ferralis et al.3 The normalized current density shown for the vertical diode structures in Fig. S7. The normalized current density reveals uniform behavior. This is similar to that of Lin et al. where they theorize this is due to thermionic emission due to the interlayer gap between WSe2 and EG.4
Figure S7: The normalized current density showing symmetrical transport for WSe2/EG devices.
3. Assignment of WSe2 Raman peaks: Low temperature (78K) Raman spectra revealed a number of peaks difficult to observe at room temperature (295K). More peaks are visible on the WSe2/EG sample as opposed to the WSe2/sapphire at 78K. Through magnification of this spectra (Fig. 2) a total of 21 peaks were observed. Table 2 identifies the preliminary peak assignments. Table 2: Preliminary peak assignments of Raman first-order and higher-order modes for 6L WSe2 based on current literature.
Peak #
Wavenumber @ 78K
Preliminary Assignment
Reference
1
84.5
Combination of shear and breathing modes
5
2
97.0
TA(M)
6
3
118.0
E1g(Γ)-LA(M) and A1g(Γ)-LA(M)
7
4
133.0
LA(M)
6–8
5
138.0
A1g(Γ)-LA(M)
6,9
6
144.0
LA(K)
7
7
222.0
Eg(K)
7
8
252.5
E12g(Γ) and A1g(Γ)
6,8,10,11
9
263.0
A1g(M) and 2LA(M)
6,7
10
~288
2LA(K)
7
11
311.0
A21g(Γ)
8,11,12
12
351.0
2Eg(Γ)
11
13
362.0
14
378.0
E1g(M)+LA(M), E1g(Γ)+LA(M), and A1g(Γ)+LA(M)
6,7
15
400.0
E1g(Γ)+LA(K) and A1g(Γ)+LA(K) and 3LA(M)
6,7
16
473.5
Eg(K)+E1g(Γ) and Eg(K)+A1g(Γ)
7
17
503.5
2E1g(Γ) and 2A1g(Γ)
7
18
515.5
2LA(M)+E1g(Γ) and 2LA(M)+A1g(Γ)
6–8
19
537.5
2LA(K)
7
20
~625
Higher-order overtones and combination modes
9
21
~740
Higher-order overtones and combination modes
9
2Eg(Γ) or A1g(M)-TA(M); 2E1g(Γ)-LA(K) and
6,7
2A1g(G)-LA(K)
References (1)
Herrera-Gómez, A.; Hegedus, A.; Meissner, P. L. Chemical Depth Profile of Ultrathin Nitrided SiO[sub 2] Films. Appl. Phys. Lett. 2002, 81, 1014.
(2)
Ahn, G.; Kim, H. R.; Ko, T. Y.; Choi, K.; Watanabe, K.; Taniguchi, T.; Hong, B. H.; Ryu, S. Optical Probing of the Electronic Interaction between Graphene and Hexagonal Boron Nitride. ACS Nano 2013, 7, 1533–1541.
(3)
Ferralis, N.; Maboudian, R.; Carraro, C. Evidence of Structural Strain in Epitaxial Graphene Layers on 6H-SiC(0001). Phys. Rev. Lett. 2008, 101, 156801.
(4)
Lin, Y.-C.; Chang, C.-Y. S.; Ghosh, R. K.; Li, J.; Zhu, H.; Addou, R.; Diaconescu, B.; Ohta, T.; Peng, X.; Lu, N.; et al. Atomically Thin Heterostructures Based on Single-Layer Tungsten Diselenide and Graphene. Nano Lett. 2014.
(5)
Zhao, Y.; Luo, X.; Li, H.; Zhang, J.; Araujo, P. T.; Gan, C. K.; Wu, J.; Zhang, H.; Quek, S. Y.; Dresselhaus, M. S.; et al. Interlayer Breathing and Shear Modes in Few-Trilayer MoS2 and WSe2. Nano Lett. 2013, 13, 1007–1015.
(6)
Zhao, W.; Ghorannevis, Z.; Amara, K. K.; Pang, J. R.; Toh, M.; Zhang, X.; Kloc, C.; Tan, P. H.; Eda, G. Lattice Dynamics in Mono- and Few-Layer Sheets of WS2 and WSe2. Nanoscale 2013, 5, 9677–9683.
(7)
Del Corro, E.; Terrones, H.; Elias, A.; Fantini, C.; Feng, S.; Nguyen, M. A.; Mallouk, T. E.; Terrones, M.; Pimenta, M. A. Excited Excitonic States in 1L, 2L, 3L, and Bulk WSe2 Observed by Resonant Raman Spectroscopy. ACS Nano 2014,
8, 9629–9635. (8)
Terrones, H.; Del Corro, E.; Feng, S.; Poumirol, J. M.; Rhodes, D.; Smirnov, D.; Pradhan, N. R.; Lin, Z.; Nguyen, M. a T.; Elías, a L.; et al. New First Order Raman-Active Modes in Few Layered Transition Metal Dichalcogenides. Sci.
Rep. 2014, 4, 4215. (9)
Li, H.; Lu, G.; Wang, Y.; Yin, Z.; Cong, C.; He, Q.; Wang, L.; Ding, F.; Yu, T.; Zhang, H. Mechanical Exfoliation and Characterization of Single- and Few-Layer Nanosheets of WSe₂ , TaS₂ , and TaSe₂. Small 2013, 9, 1974–1981.
(10) Sahin, H.; Tongay, S.; Horzum, S.; Fan, W.; Zhou, J.; Li, J.; Wu, J.; Peeters, F. M. Anomalous Raman Spectra and Thickness-Dependent Electronic Properties of WSe2. Phys. Rev. B - Condens. Matter Mater. Phys. 2013, 87, 165409. (11)
Luo, X.; Zhao, Y.; Zhang, J.; Toh, M.; Kloc, C.; Xiong, Q.; Quek, S. Y. Effects of Lower Symmetry and Dimensionality on Raman Spectra in Two-Dimensional WSe_{2}. Phys. Rev. B 2013, 88, 195313.
(12)
Ribeiro-Soares, J.; Almeida, R. M.; Barros, E. B.; Araujo, P. T.; Dresselhaus, M. S.; Cançado, L. G.; Jorio, A. Group Theory Analysis of Phonons in
Two-Dimensional Transition Metal Dichalcogenides. Phys. Rev. B 2014, 90, 115438.
Highly Scalable, Atomically Thin WSe2 grown via Metal-Organic Chemical Vapor Deposition Sarah M. Eichfeld, 1,2 Lorraine Hossain,1,2 Yu-Chuan Lin,1,2 Aleksander F. Piasecki,1,2 Benjamin Kupp,1,2 A. Glen Birdwell,4 Robert A. Burke,4 Ning Lu,3 Xin Peng,3 Jie Li,2,5 Angelica Azcatl,3 Stephen McDonnell,3 Robert M. Wallace,3 Theresa S. Mayer,2,5 Moon J. Kim,3 Joan M. Redwing,1,2 and Joshua A. Robinson1,2* 1. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, United States 2. Center for Two-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, United States 3. Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States 4. US Army Research Laboratory, Adelphi, Maryland 20783, United States 5. Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802, United States *E-mail: [email protected]
The following pages include supporting figures and experimental data. 1. Growth of WSe2: WSe2 is synthesized via MOCVD in a cold wall vertical reactor system. The gas manifold has two bubbler stations equipped with W(CO)6 and (CH3)2Se allowing for
individual control of each precursor. The bubbler pressure is held at 700 and 760 Torr respectively, and bubblers are held at room temperature. The MOCVD system is equipped with both H2 and N2 that can be used as the carrier gas.
Figure S1 a) Schematic of MOCVD process allowing for precise precursor control in a vertical cold wall system for the investigation of the synthesis conditions b) AFM of WSe2 on sapphire after growth showing monolayer WSe2 can be achieved c) Raman spectra displaying the E12g/A1g peak around 250 cm-1 indicating high quality WSe2. d) PL of monolayer WSe2 on sapphire. e) Raman map of A1g intensity showing additional layers on sapphire nucleate from edge of domains as opposed to the center when synthesized on sapphire substrates.
2. XPS of WSe2 XPS of the as grown WSe2 samples were carried out using a monochromated Al Kα source (hv = 1486.7 eV), equipped with a 7 channel analyzer. The XPS spectra were acquired at a pass energy of 15 eV and a take-off angle of 45°. An Omicron CN10 electron flood source was employed for charge neutralization during XPS scanning. XPS peak deconvolution was performed using the spectral analysis software analyzer where Voigt line shapes and an active Shirley background were used for peak fitting.1 The Se/W ratio and the WO3/WSe2 ratio were then determined (Fig. S2, T1) for synthesis using 100% H2 as the carrier gas compared to a 1:3 H2:N2 ratio. The peak positions for Se 3d and W 4f were affected by sample charging due to the insulating properties of the sapphire. This was corrected using a thin layer of Au deposited on the sample surface in addition to a charge neutralizer.
Figure S2: XPS binding energy for W 4f and Se 3d peaks comparing 100% H2 as the carrier gas to a 1:3 H2:N2 ratio. The characteristic tungsten region shows both W 4f and W 5p3/2 with the W5p3/2 appearing at 39.4 eV. Therefore, the W 5p3/2 peak overlaps with the WO3 peaks (deconvolution of such peaks are shown in the figure). The Se/W ratio was calculated using the integrated intensity of the Se 3d and W 4f peaks. Table T1 below shows a Se/W and WO3/WSe2 ratiosfor samples grown using 100% H2 versus 1:3 H2:N2 ratio. Growth with 100% H2 results in a Se/W ratio of 2, while growth with a 1:3 H2:N2 ratio results in a Se/W ratio of 1.8 which is Se deficient. The WO3/WSe2 ratio is higher for the 100% H2 however this sample was grown prior to the sample synthesized in a H2/N2 ambient. WSe2 can be highly sensitive to the air, so this could account for the slightly higher ratio of WO3/WSe2. Table T1: Se/W ratio and WO3/WSe2 ratio for samples grown using 100 % H2 as compared to samples grown using a 1:3 H2:N2 ratio. Samples
Se/W Ratio
WO3/WSe2 Ratio
100 % H2
2.0
0.25
1:3 H2:N2
1.8
0.09
The deconvolution of C incorporation in these samples is difficult due to overlap of the
carbon 1s peak with the Se Auger feature, so it is necessary to examine Raman spectra of these two samples to further determine the sample quality and if there is any carbon present in the Raman spectra (Fig S3). The PL for the sample grown using a H2/N2 ambient was quenched as opposed to strong PL for the sample grown in H2. The domain size and layer thickness was similar for both samples.
Figure S3: The impact of H2 on the growth of WSe2. (a) Raman spectra comparing 100% H2 versus 1:3 H2:N2 as the carrier gas for synthesis of WSe2. A H2:N2 mix for the carrier gas shows the carbon impurity as seen by D and G peaks. The PL is also quenched under the presence of carbon in the WSe2. The WSe2 material quality can be highly sensitive to the synthesis process including H2 concentration and precursor quality. The purity of the dimethylselenium ((CH3)2Se) was investigated, and two different precursor vendors including SAFC HiTech (99.99% purity) and STREM Chemical (99.0% purity) were utilized. The use of a lower purity source lead to carbon incorporation as shown from the Raman and PL in Figure S4.
Figure S4: The impact of the impurity in Se precursor on the WSe2 monolayer under the same growth conditions. The red line indicating a Se source purity of 99.0% and the black curve indicating a purity of 99.99%: (a) Raman spectra indicating that the Se precursor with higher impurity yielded carbon impurity incorporation in the WSe2 layers and impacts the electronic structures as evidenced by the quenched PL (b) on samples grown with lower Se precursor purity.
3. Impact of Substrate: The synthesis of WSe2 led to large differences in the final product based on choice of substrate. Initial evidence from both Raman and PL suggests there is some interaction at the WSe2/sapphire interface. Cross-sectional TEM image (Fig. S5) reveals there is some disorder present directly at the interface between the WSe2 and sapphire substrate.
Figure S5: TEM cross-section showing some disorder at WSe2/sapphire interface suggesting possible interaction between WSe2 and the sapphire substrate.
Shifts in the PL peak position for WSe2 on CVD graphene suggests there is some strain in the layer. This was further investigated by examining shifts in the Raman 2D and G peaks in Fig. S6a. The data are vector decomposed (Fig. S6b) to correlate peak shifting to tensile and compressive strain (eT and eC, respectively), Fermi velocity reduction (eFVR), and hole doping (eH), using methods by Ahn et al.2
Figure S6: The presence of strain in WSe2 on CVD graphene. a.) Raman spectra of CVD graphene compared to WSe2 on CVD graphene normalized to the SiO2 at 520 cm-1 showing significant G and 2D peak shifts to higher frequency. b.) Plot of Raman 2D frequency vs. G peak frequency for CVD graphene on SiO2 (black) compared to annealed CVD graphene/SiO2 (blue) and WSe2 growth on CVD Graphene/SiO2 (red). The WSe2 growth resulted in 0.4% compressive strain, while the same growth condition without W and Se sources introduced resulted in 0.2 % compressive strain comparing to a (freestanding graphene sample). This indicates that the WSe2 deposited on CVDGr contributes additional 0.2% strain in addition to the strain from the thermal effects. The strain in monolayer CVDGr is calculated according to Ferralis et al.3 The normalized current density shown for the vertical diode structures in Fig. S7. The normalized current density reveals uniform behavior. This is similar to that of Lin et al. where they theorize this is due to thermionic emission due to the interlayer gap between WSe2 and EG.4
Figure S7: The normalized current density showing symmetrical transport for WSe2/EG devices.
3. Assignment of WSe2 Raman peaks: Low temperature (78K) Raman spectra revealed a number of peaks difficult to observe at room temperature (295K). More peaks are visible on the WSe2/EG sample as opposed to the WSe2/sapphire at 78K. Through magnification of this spectra (Fig. 2) a total of 21 peaks were observed. Table 2 identifies the preliminary peak assignments. Table 2: Preliminary peak assignments of Raman first-order and higher-order modes for 6L WSe2 based on current literature.
Peak #
Wavenumber @ 78K
Preliminary Assignment
Reference
1
84.5
Combination of shear and breathing modes
5
2
97.0
TA(M)
6
3
118.0
E1g(Γ)-LA(M) and A1g(Γ)-LA(M)
7
4
133.0
LA(M)
6–8
5
138.0
A1g(Γ)-LA(M)
6,9
6
144.0
LA(K)
7
7
222.0
Eg(K)
7
8
252.5
E12g(Γ) and A1g(Γ)
6,8,10,11
9
263.0
A1g(M) and 2LA(M)
6,7
10
~288
2LA(K)
7
11
311.0
A21g(Γ)
8,11,12
12
351.0
2Eg(Γ)
11
13
362.0
14
378.0
E1g(M)+LA(M), E1g(Γ)+LA(M), and A1g(Γ)+LA(M)
6,7
15
400.0
E1g(Γ)+LA(K) and A1g(Γ)+LA(K) and 3LA(M)
6,7
16
473.5
Eg(K)+E1g(Γ) and Eg(K)+A1g(Γ)
7
17
503.5
2E1g(Γ) and 2A1g(Γ)
7
18
515.5
2LA(M)+E1g(Γ) and 2LA(M)+A1g(Γ)
6–8
19
537.5
2LA(K)
7
20
~625
Higher-order overtones and combination modes
9
21
~740
Higher-order overtones and combination modes
9
2Eg(Γ) or A1g(M)-TA(M); 2E1g(Γ)-LA(K) and
6,7
2A1g(G)-LA(K)
References (1)
Herrera-Gómez, A.; Hegedus, A.; Meissner, P. L. Chemical Depth Profile of Ultrathin Nitrided SiO[sub 2] Films. Appl. Phys. Lett. 2002, 81, 1014.
(2)
Ahn, G.; Kim, H. R.; Ko, T. Y.; Choi, K.; Watanabe, K.; Taniguchi, T.; Hong, B. H.; Ryu, S. Optical Probing of the Electronic Interaction between Graphene and Hexagonal Boron Nitride. ACS Nano 2013, 7, 1533–1541.
(3)
Ferralis, N.; Maboudian, R.; Carraro, C. Evidence of Structural Strain in Epitaxial Graphene Layers on 6H-SiC(0001). Phys. Rev. Lett. 2008, 101, 156801.
(4)
Lin, Y.-C.; Chang, C.-Y. S.; Ghosh, R. K.; Li, J.; Zhu, H.; Addou, R.; Diaconescu, B.; Ohta, T.; Peng, X.; Lu, N.; et al. Atomically Thin Heterostructures Based on Single-Layer Tungsten Diselenide and Graphene. Nano Lett. 2014.
(5)
Zhao, Y.; Luo, X.; Li, H.; Zhang, J.; Araujo, P. T.; Gan, C. K.; Wu, J.; Zhang, H.; Quek, S. Y.; Dresselhaus, M. S.; et al. Interlayer Breathing and Shear Modes in Few-Trilayer MoS2 and WSe2. Nano Lett. 2013, 13, 1007–1015.
(6)
Zhao, W.; Ghorannevis, Z.; Amara, K. K.; Pang, J. R.; Toh, M.; Zhang, X.; Kloc, C.; Tan, P. H.; Eda, G. Lattice Dynamics in Mono- and Few-Layer Sheets of WS2 and WSe2. Nanoscale 2013, 5, 9677–9683.
(7)
Del Corro, E.; Terrones, H.; Elias, A.; Fantini, C.; Feng, S.; Nguyen, M. A.; Mallouk, T. E.; Terrones, M.; Pimenta, M. A. Excited Excitonic States in 1L, 2L, 3L, and Bulk WSe2 Observed by Resonant Raman Spectroscopy. ACS Nano 2014,
8, 9629–9635. (8)
Terrones, H.; Del Corro, E.; Feng, S.; Poumirol, J. M.; Rhodes, D.; Smirnov, D.; Pradhan, N. R.; Lin, Z.; Nguyen, M. a T.; Elías, a L.; et al. New First Order Raman-Active Modes in Few Layered Transition Metal Dichalcogenides. Sci.
Rep. 2014, 4, 4215. (9)
Li, H.; Lu, G.; Wang, Y.; Yin, Z.; Cong, C.; He, Q.; Wang, L.; Ding, F.; Yu, T.; Zhang, H. Mechanical Exfoliation and Characterization of Single- and Few-Layer Nanosheets of WSe₂ , TaS₂ , and TaSe₂. Small 2013, 9, 1974–1981.
(10) Sahin, H.; Tongay, S.; Horzum, S.; Fan, W.; Zhou, J.; Li, J.; Wu, J.; Peeters, F. M. Anomalous Raman Spectra and Thickness-Dependent Electronic Properties of WSe2. Phys. Rev. B - Condens. Matter Mater. Phys. 2013, 87, 165409. (11)
Luo, X.; Zhao, Y.; Zhang, J.; Toh, M.; Kloc, C.; Xiong, Q.; Quek, S. Y. Effects of Lower Symmetry and Dimensionality on Raman Spectra in Two-Dimensional WSe_{2}. Phys. Rev. B 2013, 88, 195313.
(12)
Ribeiro-Soares, J.; Almeida, R. M.; Barros, E. B.; Araujo, P. T.; Dresselhaus, M. S.; Cançado, L. G.; Jorio, A. Group Theory Analysis of Phonons in
Two-Dimensional Transition Metal Dichalcogenides. Phys. Rev. B 2014, 90, 115438.
