2RD, United Kingdom. 3Institute for Problems of Materials Science, The National Academy of Sciences of Ukraine,. Chernivtsi Branch, Chernivtsi, 58001 Ukraine.
SUPPORTING INFORMATION Engineering p-n Junctions and Bandgap Tuning of InSe Nanolayers by Controlled Oxidation
Nilanthy Balakrishnan1*, Zakhar R. Kudrynskyi1, Emily F. Smith2, Michael W. Fay2, Oleg Makarovsky1, Zakhar D. Kovalyuk3, Laurence Eaves1, Peter H. Beton1, Amalia Patanè1*
1
School of Physics and Astronomy, The University of Nottingham, Nottingham NG7 2RD,
United Kingdom 2
Nanoscale and Microscale Research Centre, The University of Nottingham, Nottingham NG7
2RD, United Kingdom 3
Institute for Problems of Materials Science, The National Academy of Sciences of Ukraine,
Chernivtsi Branch, Chernivtsi, 58001 Ukraine
Keywords: two dimensional materials, indium selenide, van der Waals crystals, indium oxide
S1: Photo-annealing studies at constant laser power Figure S1a shows the dependence on the laser exposure time, ta, of the PL peak energy for p-InSe flakes with different layer thickness L (λa = 532 nm and Pa = 1 mW). The PL peak energy position, E2D, of bulk flakes (L = 15 nm) is not affected by laser exposure for ta up to 480 s. However, thin flakes (L ≤ 7 nm) exhibit a systematic energy blue-shift with increasing ta. Figure S1b shows the normalized room temperature PL spectra of p-InSe flakes of different L before and after laser exposure for a time ta = 10 s and 480 s. We find that a laser power Pa = 1 mW and ta > 10 s are sufficient to blue-shift the PL emission of thin (L ≤ 7 nm) InSe flakes.
(b) Normalized PL Intensity
(a) L = 5 nm
1.50
L = 6 nm
E2D (eV)
1.45
L = 7 nm
1.40
L = 15 nm
1.28
0
100
200
300
ta (s)
400
500
1.2
Freshly exfoliated ta = 10 s ta = 480 s L = 5 nm
L = 6 nm
L = 7 nm L = 15 nm
1.4
1.6
Energy (eV)
1.8
Figure S1: (a) Room temperature PL peak energy, E2D, of p-InSe flakes of different thickness L versus annealing time ta. The photo-annealing is conducted with a laser of power Pa = 1 mW and wavelength λa = 532 nm. (b) Normalized PL spectra of p-InSe flakes with different L before and after laser exposure for a time ta = 10 s and 480 s. The PL measurement is conducted at low laser power (< 0.1 mW).
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S2: X-Ray photoelectron spectroscopy (XPS) of p-InSe nanolayers We performed XPS on freshly exfoliated and thermally annealed p-InSe flakes (Figures S2a-b). The substrate (SiO2/Si) signal was observed from the gaps between the flakes. The stoichiometric composition of the freshly exfoliated layers is [In] = 51 1 atomic % and [Se] = 49 1 atomic %. The binding energy of In 4d5/2 is EIn = 17.6 eV, see Figure S2b. Following the thermal annealing at Ta = 175 oC for ta = 1 hr, the stoichiometric composition changes to [In] = 88 1 atomic % and [Se] = 12 1 atomic %. Moreover, In 4d spectra shows two
XPS Intensity (arb.units)
additional peaks at EIn = 18.3 eV and 19.2 eV, which correspond to In2O3.[1] (a)
O 1s
Freshly exfoliated
Si 2p O 2s Si 2s
O KLL o
Annealed at Ta = 175 C
1400 1200 1000
800
Se 3d In 4d
In 3d
In 3p
C KLL
C 1s
600
400
200
0
XPS Intensity (arb. units)
Binding Energy (eV)
(b)
Freshly exfoliated
In 4d5/2
In 4d3/2
(17.6 eV)
(18.5 eV)
o
Annealed at Ta = 175 C
In2O3 20
18
Binding Energy (eV)
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Figure S2. XPS spectra (a) and high-resolution In 4d spectra (b) of freshly exfoliated flakes (top) and flakes annealed at Ta = 175 oC for ta = 1 hr (bottom). Red and blue curves in (b) are pseudo-Voigt functions (sum of 80% -Gaussian and 20 % -Lorentzian) fitted (green curve) to the measured XPS spectra (black curve). 3
S3: Time-dependent thermal annealing studies at constant temperature The PL peak energy position of bulk InSe flakes (L > 15 nm) is not affected by annealing at Ta = 125 C for ta up to 420 mins. However, thin flakes (L < 8 nm) tend to blue-shift by up to ~ 15 meV (Figures S3a-c). Figure S3d shows the layer thickness, L*, of the non-oxidized InSe layer following the annealing, as estimated using the measured PL peak energy and the half-infinite quantum well model described in the main text. L* decreases rapidly with increasing ta (Figure S3e). Freshly exfoliated 30 mins 60 mins 120 mins 240 mins 420 mins
L = 23.1 nm
1000
5 mm 500
0
1.1
1.2
1.3
1.4
(b) 20
10
0
1.5
1.3
1.4
Energy (eV)
(d) L = 4.5 nm L = 6.2 nm
10
(e) L = 14.1 nm L = 23.1 nm
0 0
120
240
1.6
1.7
360
Ta = 125 oC
10 ta = 420 mins 5 5
L (nm)
10
2.8
15
Ta = 125 oC L = 4.5 nm
2.7
*
5
15
0 0
L = 8.6 nm
L (nm)
Eh (meV)
15
*
Ta = 125 oC
1.5
Energy (eV)
L (nm)
(c)
Freshly exfoliated 30 mins 60 mins 120 mins 240 mins 420 mins
L = 4.5 nm
PL Intensity (arb.units)
PL Intensity (arb.units)
(a)
2.6 0
120
ta (mins)
240
360
ta (mins)
Figure S3. (a-b) PL spectra of exfoliated p-InSe layers with different layer thickness L (T = 300 K, P = 0.1 mW, = 633 nm). Spectra were measured following an annealing at Ta = 125 o
C and increasing annealing times ta. (c) ta-dependence of the energy blue-shift of the PL
emission for different L. (d) L-dependence of the thickness of the non-oxidized InSe layer, L*, following an annealing at Ta = 125 oC and ta = 420 mins. The dashed line represents the thickness of the flake before annealing. (e) ta-dependence of L*. 4
S4: HRTEM and CBED of p-InSe/n-In2O3 junction device To fabricate p-InSe/n-In2O3 junction devices, bulk flakes of p-InSe with area of ~ 5×5 mm2 and thickness of ~ 1 mm were annealed in air at Ta = 450 oC for ta = 96 hours. The crosssectional TEM image (Figure S4a) shows a Se-rich layer intermediate between the In2O3 and InSe layers. The EDX maps of the heterstructure are shown in Figure 7 in the main text. The high resolution TEM (HRTEM) and convergent beam electron diffraction (CBED) images of In2O3, Se-rich InSe, and InSe regions reveal that all three regions are highly-crystalline.
Figure S4. (a) TEM image of a cross-sectional area of the p-InSe/n-In2O3 junction. The yellow line shows the top surface of In2O3. HRTEM (top) and CBED (bottom) images of the In2O3 (b), Se-rich InSe (c), and InSe (d) layers of the p-InSe/n-In2O3 junction, respectively.
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Table S1. Electron and hole effective masses of InSe and In2O3 for motion along the c-axis as reported in references 2-5. ∗ ݉ ||
∗ ݉ ||
ߤ||
InSe
0.08 me[2]
0.17 me[3]
0.054 me
In2O3
0.3 me[4]
0.79 me[5]
0.217 me
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