Supporting Information
Red to Ultraviolet Emission Tuning of Two Dimensional Gallium Sulfide/Selenide
Chan Su Jung,† Fazel Shojaei,‡ Kidong Park,† Jin Young Oh,† Hyung Soon Im,† Dong Myung Jang,† Jeunghee Park,*,† and Hong Seok Kang*,§
†
‡
Department of Chemistry, Korea University, Jochiwon 339-700, Korea Department of Chemistry and Bioactive Material Sciences and Research Institute of Physics and
Chemistry, Jeonbuk National University, Jeonju 560-756, Korea §
Department of Nano and Advanced Materials, College of Engineering, Jeonju University, Jeonju 560-
759, Korea
Contents I. Supporting Tables: Tables S1-S4 II. Supporting Figures: Figures S1-S11
S1
I. Supporting Tables
Table S1. Structural parameters of GaS and GaSe (monolayer, bilayer, and trilayer) calculated using both HSE06 and PBE-D2 functionals. HSE06
GaS
No. of Layers 1
2
3
GaSe
1
2
3 (phase) 3 (phase) a
PBE-D2
Stretch (%) 0 1 5 0 1 5 0 1 5
La
lGa-Ga
lGa-S
lS-S
La
lGa-Ga
lGa-S
lS-S
3.580 3.616 3.759 3.580 3.616 3.759 3.580 3.616 3.759
2.45 2.45 2.45 2.42 2.42 2.42 2.42 2.42 2.42
2.33 2.34 2.38 2.33 2.34 2.38 2.33 2.34 2.38
4.61 2.56 4.44 4.57 4.52 4.42 4.57 2.52 4.42
3.580 3.616 3.759 3.580 3.616 3.759 3.580 3.616 3.759
2.44 2.44 2.45 2.44 2.45 2.45 2.44 2.45 2.45
2.35 2.36 2.40 2.35 2.36 2.40 2.35 2.36 2.40
2.67 2.64 2.50 4.66 4.64 4.50 4.66 2.63 4.50
0 1 5 0 1 5 0 1 5 0 1 5
3.750 3.788 3.938 3.750 3.788 3.938 3.750 3.788 3.938 3.750 3.788 3.938
2.44 2.45 2.45 2.41 2.41 2.41 2.41 2.41 2.41 2.41 2.41 2.41
2.46 2.47 2.52 2.45 2.46 2.52 2.45 2.46 2.52 2.45 2.46 2.52
4.77 4.74 4.59 4.70 4.67 4.52 4.70 4.67 4.52 4.70 4.67 4.52
3.750 3.788 3.938 3.755 3.793 3.943 3.752 3.790 3.940 3.752 3.790 3.940
2.44 2.45 2.45 2.43 2.43 2.44 2.43 2.44 2.45 2.43 2.44 2.44
2.47 2.49 2.53 2.48 2.49 2.53 2.48 2.49 2.53 2.48 2.49 2.53
4.83 4.81 4.66 4.82 4.79 4.65 4.82 4.79 4.66 4.82 4.79 4.66
Lattice constant a.
S2
Table S2. Band gaps for optimized mono-, bi-, and trilayer configurations of GaS and GaSe, calculated using PBE-D2 functional (all energies in eV).
GaS
No. of Layers 1
2
3
GaSe
L (Å)a
Direct (→)
Indirect
3.580
2.881
2.580 ((K-)→M) 2.586 ((-M)→M) 2.705 (→M) 2.129 ((K-)→M)) 2.130((-M)→M) 2.159 (→M) 1.924 ((K-)→M) 1.924 ((-M)→M) 1.932 (→M) 2.202 ((K-)→) 2.207 ((-M)→) 1.611 ((K-)→) 1.613 ((-M)→) 1.383 ((-M)→) 1.384 ((K-)→) 1.420 ((K-)→) 1.420 ((-M)→)
3.580
3.580
2.364
2.097
1
3.750
2.309
2
3.755
1.663
3 (phase) 3 (phase)
3.752
1.412
3.752
1.445
Egb
Ebc (eV/layer)
0.301
0
0.244
-0.67
0.173
-0.92
0.107
0
0.052
-0.95
0.029
-1.26
0.025
-1.27
The lattice constant a; b ∆Eg = Eg (direct) – Eg (indirect); energy difference between the direct and indirect band gaps. c Binding energy of layers, which is equated to its total energy with the optimized lattice constant. a
S3
Table S3. Band gaps and relative stabilities Erel for various configurations of GaS1-xSex monolayer at x = 0.25, 0.5, and 0.75, calculated using PBE-D2 functional. x (Se)
Configuration
Erel. (eV/supercell)
Direct (→)
Indirecta
Stabilityb
2.487 (K-)→ O 2.492 (-M)→ 0.25 2.531 (K-)→ B 0.001 2.647 O 2.537 (-M)→ C 0.027 X (K-)→ A 0 2.477 O (-M)→ 2.383 (K-)→ B 0.001 2.496 O 0.5 2.390 (-M)→ 2.429 (K-)→ C 0.001 2.542 O 2.434 (-M)→ D 0.102 X 2.276 (K-)→ A 0 2.386 O 2.281 (-M)→ 0.75 2.269 (K-)→ B 0.001 2.379 O 2.275 (-M)→ C 0.022 X a Corresponding transition in the k-space; b O and X represents the stable and unstable configurations. The band gaps are calculated for only stable configurations. A
0
2.601
We have investigated the band structures of GaS1-xSex alloy monolayer with x = 0.25, 0.5, and 0.75, using PBE-D2 functional. We can expect that the structure of the alloy monolayer consists of various local domains with specific stable configurations. As a first step, we identify the most stable configurations built using a supercell (22 cell), which are stable within 1 meV per supercell. Figure S-a shows the structures of four possible configurations (A-D) of GaS0.5Se0.5 monolayer; A, B, and C, are almost equally stable, while D is unstable. In the stable configurations (A-C), S and Se atoms are distributed in two sublayers; (S,Se)Ga-Ga-(S,Se). The configuration D has S or Se atoms exclusively in one of two sublayer; SGa-Ga-Se. Figure S-b shows the indirect/indirect band gaps (PBE-D2) of GaS1-xSex alloy monolayer with the stable configurations (A-C). We performed the calculations for the A-C configurations of GaS0.5Se0.5 monolayer using HSE06 functional, and plotted the band gap values (see Table 1 in text) in Figure S-c. For all alloy composition, the small difference between the direct and indirect band gaps; ~0.1 eV (for both PBE-06 and HSE06 functional calculation), is comparable to that of GaSe. S4
Figure S-a. Four possible configurations (A-D) of GaS0.5Se0.5 monolayer considered in this work Configuration A
Configuration B
S5
Configuration C
Configuration D
S6
Figure S-b. Direct and indirect band gaps of GaS1-xSex monolayer (A, B, and C configurations) as functions of x, calculated using PBE-D2 functional. The average value for the band gaps of configurations at each x is connected with the band gap of GaS and GaSe, by a line. 3.0
Monolayer (PBE-D2) A B C Average
GaS
Band Gap (eV)
2.8
2.6
Direct
2.4
GaSe Indirect
2.2
2.0
0.0
0.2
0.4
0.6
0.8
1.0
x (Se)
Figure S-c. Direct and indirect band gaps of GaS, GaS0.5Se0.5, and GaSe monolayers calculated using HSE06 functional. For GaS0.5Se0.5, three equally stable configurations (A, B, and C) are considered. See the text for their numerical values. The average value for the band gaps of A-C configurations is connected with the band gap of GaS and GaSe, by a line. Monolayer (HSE06)
Band Gap (eV)
4.0
A B C Average
GaS
4.0
3.5
GaS0.5Se0.5
3.5
Direct GaSe
3.0
3.0
2.5
Indirect 2.0 2.5
0.0
0.5
1.0
x (Se) S7
0.0
0.5
1.0
Table S4. Band gaps for mono- (1 L), bi- (2 L), and trilayer (3 L) configurations of GaS and GaSe as a function of lattice constant a (Å), calculated using (a) HSE06 and (b) PBE-D2 functionals (all energies in eV). (a) HSE06
GaS
1L
2L
3L
GaSe
1L
Strain (%)
L (Å)a
Direct (→)
0
3.580
3.855
0.5
3.598
3.646
1
3.616
3.509
1.6
3.640
3.327
5
3.759
2.594
6.8
3.822
2.179
0
3.580
3.346
1
3.616
3.027
5
3.759
2.117
0
3.580
3.133
1
3.616
2.822
5
3.759
1.913
0
3.750
3.093
0.3
3.765
2.970
1
3.788
2.828
1.9
3.820
2.612
5
3.938
1.939
7.0
4.011
1.606 S8
Indirect 3.325 ((K-)→M) 3.330 ((-M)→M) 3.433 (→M), 3.219 ((K-)→M) 3.223 ((-M)→M) 3.320 (→M) 3.210 ((K-)→M)) 3.214 ((-M)→M) 3.311 (→M) 3.151 ((K-)→M)) 3.154 ((-M)→M) 2.467 ((K-)→) 2.471 ((-M)→) 2.009 ((K-)→) 2.015((-M)→) 2.861 ((K-)→M) 2.862 ((-M)→M) 2.881 (→M) 2.749 ((-M)→M) 2.749 ((K-)→M)) 2.769 (→M) 2.076 ((-M)→) 2.077 ((K-)→) 2.654 ((K-)→M) 0.48 (-M)→M) 2.657 (→M) 2.560 ((-M)→M) 2.560 ((-M)→M) 2.563 (→M) 1.903 ((K-)→M)) 1.904 ((-M)→) 3.001 ((K-)→) 3.002 ((-M)→) 2.873 ((K-)→) 2.875 ((-M)→) 2.732 ((-M)→) 2.735 ((K-)→) 2.513 ((-M)→) 2.515 ((K-)→) 1.798 ((-M)→) 1.802 ((K-)→) 1.414((-M)→)
Egb 0.530
0.427
0.299 0.176 0.127 0.170 0.485
0.278 0.041 0.479
0.262 0.100 0.092 0.097 0.096 0.099 0.141 0.192
1.423 ((K-)→) 0.024 2.426 ((K-)→) 0 3.750 2.455 2L 2.427 ((-M)→) 2.545 ((-)→) 0.029 2.192 ((-M)→) 1 3.788 2.221 2.193 ((K-)→) 0.060 1.299 ((-M)→) 5 3.938 1.359 1.299 ((K-)→) 0.008 2.173 ((K-)→) 0 3.750 2.181 3 L () 2.173 ((-M)→) 0.008 1.951((-M)→) 1 3.788 1.959 1.951 ((K-)→) 0.030 1.058 ((-M)→) 5 3.938 1.088 1.058 ((K-)→) 0.005 2.199 ((K-)→) 0 3.750 2.204 3 L () 2.199 ((-M)→) 0..007 2.000 ((-M)→) 1 3.788 2.007 2.000 ((K-)→) 0.013 1.105 ((-M)→) 5 3.938 1.128 1.106 ((K-)→) a The lattice constant a; b ∆Eg = Eg (direct) – Eg (indirect); energy difference between the direct and indirect band gaps.
(b) PBE-D2
GaS
1L
2L
3L
Strain (%)
L (Å)a
Direct (→)
0
3.580
2.881
1.7
3.640
2.459
2.7
3.676
2.233
6.8
3.822
1.444
0
3.580
2.364
1
3.616
3.027
5
3.759
2.117
0
3.580
2.097
1
3.616
1.927 S9
Indirect 2.580 ((K-)→M) 2.586 ((-M)→M) 2.705 (→M) 2.351 ((K-)→) 2.356 ((-M)→) 2.484 ((K-)→) 2.124 ((K-)→M)) 2.129 ((-M)→M) 2.420 ((K-→M) 1.279 ((K-)→) 1.286 ((-M)→) 2.129 ((K-)→M) 2.130 ((-M)→M) 2.159 (→M) 2.078 ((-M)→M) 2.078 ((K-)→M)) 20.107 (→M) 1.127 ((-M)→) 1.127 ((K-)→) 1.924 ((K-)→M) 1.924 (-M)→M) 1.932 (→M) 1.887 ((-M)→M)
Egb
∆Esc
0.301
0
0.108
0.002
0.109
0.003
0.165
0.040
0.211
0
0.235
0.002
0.99
0.044
0.146
0
0.040
0.002
GaSe
1L
2L
3 L ()
3 L ()
a
5
3.759
1.157
0
3.750
2.309
1.9
3.820
1.911
2.9
3.858
1.697
7.0
4.011
1.026
0
3.755
1.663
1
3.793
1.464
5
3.943
0.762
0
3.752
1.412
1
3.790
1.217
5
3.938
0.538
0
3.752
1.455
1
3.790
1.251
5
3.938
0.511
1.887 (→M) 1.127 (()→)) 1.127 ((K-)→) 2.202 ((K-)→) 2.207 ((-M)→) 1.805 ((K-)→) 1.810 ((-M)→) 1.584 ((-M)→) 1.589 (()→) 0.838 ((K-)→) 0.845 (()→) 1.611 ((K-)→) 1.613 ((-M)→) 1.410 ((K-)→) 1.410 (()→) 0.655 ((K-)→) 0.657 (()→) 1.383 (()→) 2.173 ((K-)→) 1.188 ((-M)→) 1.188 ((K-)→) 0.512 ((-M)→) 0.512 ((K-)→) 1.420 ((K-)→) 1.420 ((-M)→) 1.225 ((K-)→) 1.255 ((-M)→) 0.475 ((K-)→) 0.475 ((-M)→)
0.030
0.044
0.107
0
0.106
0.002
0.113
0.002
0.188
0.038
0.052
0
0.054
0.002
0.107
0.044
0.029
0
0.029
0.002
0.026
0.043
0.035
0
0.026
0.002
0.036
0.041
The lattice constant a; b∆Eg = Eg (direct) – Eg (indirect); energy difference between the direct and
indirect band gaps; c∆Es (eV/atom) = Etot (x %) - Etot (0 %); the strain energy, which is defined as the total energy difference between the strained and unstrained and structures. At 1% strain, the strain energy for mono-, bi-, and trilayer graphene is 0.008, 0.008, and 0.009 eV, respectively, while the corresponding energy is 0.157, 0.160, and 0.162 eV at 5% strain.
S10
II. Supporting Figures Figure S1. (a) Full-range XRD patterns and (b) (004) peak regions of as-grown GaS1-xSex multilayered nanosheets. The reference peaks of -GaS (top), -GaSe and rhombohedral (R) phase -GaSe (bottom) are also displayed; -GaS (JCPDS No: 30-0576; a = 3.587 Å and c = 15.492 Å), -GaSe (JCPDS No: 03-65-3508; a = 3.755 Å and c = 15.94 Å), -GaSe (a = 3.755 Å and c = 23.92 Å). The GaS peaks correspond well to -GaS. As x increases, the peaks shift from those of GaS to those of GaSe, followed by the phase conversion from pure -GaS to a mixture of - and -GaSe phases. The composition (x) was determined using
(004)
(b) -GaS
(110)
(107)
(106) (008)
(105)
(104)
(006)
(101) (102) (103)
(100)
(a)
(004)
Vegard’s law (i.e., d = (1−x)dGaS+xdGaSe) based on the peak position of the end members.
30-0576
GaS GaS0.9Se0.1 GaS0.8Se0.2
Intensity (arb. units)
GaS0.7Se0.3 GaS0.6Se0.4 GaS0.5Se0.5
* * * * * *
GaS0.4Se0.6 GaS0.3Se0.7
* * * * * *
GaS0.2Se0.8 GaS0.1Se0.9
20
30
( : -GaSe)
GaSe
40
(004) (006)
(008) (1010) (110) (110) (0111) (107)
(107) (105) (018)
*
(104)
* * *
(101) (100) (101) (012) (104) (103) (015)
(006) (004)
* * *
50
60
2 (Degree)
S11
22.0
-GaSe 03-65-3508 -GaSe
22.5
23.0
Figure S2. Kubelka–Munk (K–M) transformation, the plot of (a) [F()h]1/2 and (b) [F()h]2 (where F() is the diffuse reflectance) versus photon energy h (eV), yielding the indirect and direct band gaps from the linear interpolation, respectively
(b)
2.8
2.6
2.4
2.2
[F(R)hv]2
x =1
x = 0.5
x=0
[F(R)hv]1/2
(a)
2.0
3.0
Photon Energy (eV)
2.8
2.6
2.4
2.2
Photon Energy (eV)
S12
2.0
Figure S3. (a) SEM images of the GaS monolayers grown on a large area SiOx/Si substrates. The dimension of the substrates is marked. SEM EDX spectra of selected triangles of (b) GaS and (c) GaSe. The composition was calculated using the Ga L shell, S K shell, and Se L shell peaks, confirming 1:1 ratio of Ga and S (or Se).
S13
Figure S4. Raman spectra of (a) GaS, (b) GaSe, and (c) GaS0.5Se0.5 multilayers, and epitaxially grown atomically thin layers. An excitation wavelength of 514 nm was provide from Ar-ion laser. As the laser spot moved to the thinner layers (321), the Raman scattering peaks were monitored.
The GaS multilayers show an A11g mode at 189 cm-1, E12g mode at 292 cm-1, and A21g mode at 360 cm-1.S1,S2 The two A1g modes correspond to the out-of-plane vibrational mode of the SGa-Ga-S lattice, whereas the E2g mode is associated with the in-plane vibrational mode. As the number of layers decreases to one, the A11g and A21g modes experience a small red-shift (3 cm-1) to 186 cm-1 and 358 cm-1, respectively. At position 3, the E12g mode appears at 295 cm-1 with a 3 cm-1 blue shift. As the number of layers decreases to one, the blue shifted E12g peak cannot be identified because it overlaps with the Raman peaks of the SiOx/Si substrates (303 cm-1). This result is consistent with what has been seen in the past with GaS and MoS2.S3,S4 Raman spectrum of GaSe multilayers consists of characteristic peaks of the E12g mode (211 cm-1), E21g mode (245 cm-1), and A21g mode (306 cm-1), which have overlaps with the Raman peaks of SiOx/Si substrates (303 cm-1). When approaching a monolayer (321), the Raman scattering signals from the E21g and E12g modes disappear. The intensity of the A21g S14
mode peak also decreases significantly, so in fact that the red shift is not clear because it overlaps with the Raman peaks of the substrate. The ratio of A1g and E21g also diminishes with a decreasing number of layer. The GaS0.5Se0.5 multilayers exhibit both GaS and GaSe peaks; GaS A11g mode at 170 cm-1, GaSe E12g mode at 215 cm-1, GaSe E21g modes at 234 and 260 cm-1, GaS E12g mode at 295 cm1
, GaSSe A21g mode at 337 cm-1. The peak position of the GaSSe A21g mode is consistent with
the middle value of those of the GaS and GaS A21g modes; (360+306)/2 = 333 cm-1. As the thickness decreases, the GaS A11g mode shows a 3 cm-1 red shift, whereas the GaSe E12g and GaSe E21g modes disappear. The GaSSe A21g modes shows a red shift to 334 cm-1 by 3 cm-1.
S15
Figure S5. Band structures of (a) 1%- and (b) 5%-strained GaS monolayer, (c) 1%- and (d) 5%- strained GaS bilayer, (e) 1%- and (f) 5%-strained GaS trilayer, calculated using the HSE06 functional. The Fermi level is set to energy zero.
S16
Figure S6. Band structures of (a) 1%- and (b) 5%-strained GaSe monolayer; (c) 1 %- and (d)
5%- stretched GaSe bilayer; (e) 1%- and (f) 5%-strained -GaSe trilayer; (g) 1%- and (h) 5%-strained -GaSe trilayers, calculated using the HSE06 functional. The Fermi level is set to energy zero.
S17
Figure S7. Band gap of (a) -GaS and (b) -GaSe mono- (1 L), bi- (2 L), and trilayers (3 L) as
a function of strain, calculated using PBE-D2 functional.
Band Gap (eV)
3.0
2.5
(a) GaS 1 L (Direct) 1 L (Indirect) 2 L (Direct) 2 L (Indirect) 3 L (Direct) 3 L (Indirect)
2.5
(b) GaSe 1 L (Direct) 1 L (Indirect) 2 L (Direct) 2 L (Indirect) 3 L (Direct) 3 L (Indirect)
2.0
1.5
2.0
1.0
1.5
0.5
1.0 0
1
2
3
4
5
6
7
0
1
Stain (%)
2
3
4
5
6
7
Stain (%)
Figure S8. PL spectra of belt-type GaSe multilayers, measured at both straight and bent regions, which shows a maximum red shift of the emission peak (50 meV) between the two regions. The estimated strain at the curvature is 2-3 %.
Intensity (arb. units)
1 2 3 4 5 6
624 nm (1.98 eV)
641 nm (1.93 eV)
. 580
600
620
640
Wavelength (nm)
S18
660
680
Figure S9. In-plane X-ray diffraction (XRD) pattern of GaS and GaSe monolayers. The two peaks at 2θ = 28.74° and 50.91° for GaS are assigned to (100) and (110) reflections, respectively. The (100) and (110) peaks of GaSe appear at 2θ = 28.74° and 50.91°. The lattice parameter a of GaS monolayer is calculated to be 3.603 Å, which corresponds to 0.5 % expansion from that of the multilayers (a = 3.587 Å). The GaSe monolayer also has a 0.4 % expanded lattice parameter a = 3.770 Å (vs. a =3.755 Å).
-GaS (30-0576)
-GaSe (03-65-3508)
GaSe Monolayers
27
40
45
50
(107)
(101)
(110)
(107)
55
(110)
35 (101)
30 (100)
25
(100)
Intensity (arb. units)
GaS Monolayers
28
29
30
48
2 (Degree)
S19
49
50
2 (Degree)
51
Figure S10. (a) Two different configurations (B and T) of an oxygen atom (red ball) dissociatively adsorbed on a 4 4 monolayer. The green and yellow balls represent Ga and X (X= S, Se), respectively. (b) Electronic band structures of 1OB/(66 GaS). (a)
(b)
The adsorption energy (Ead) of half an O2 molecule (i.e., one oxygen atom) on a 44 cell monolayer was calculated using PBE-D2 functional. It is assumed here that adsorption involves a complicated reaction pathway in which O2 initially physically adsorbs and migrates as oxygen atoms to different sulfur (S) sites. For GaS, two different adsorption configurations (denoted as B and T) were considered. The most stable bridge-type Configuration B is characterized by an oxygen atom (OB) bridging the Ga and S atoms. The Ead value in this case is -0.16 eV, indicating that the oxygen atom is adsorbed on the GaS with an appreciable energy release. The S-O and O-Ga bond lengths are 1.60 Å and 1.92 Å, respectively, and the S-O bond length is greater than either both a S=O double bond (= 1.49 Å) and partial double bond (= 1.43 Å).S5 The O-Ga bond length is comparable to that of a partial single bond with a bond order of 0.35 (=1.93 Å).S6 The top configuration (T), in which the oxygen atom (OT) is located on top of a P atom in such a way that a double S=O bond with a bond length of 1.50 Å is formed, is less stable by 0.08 eV. We can therefore conclude that oxygen atom will be predominantly adsorbed on GaS with a bridge configuration. For GaSe, the Ead value is -0.07 eV for Configuration B, indicating that although oxygen atom can adsorbed on the 44 cell of GaSe, this is weaker than on GaS. The weaker Se-O-Ga bridge bonds than the S-O-Ga bonds is responsible for its instability. Configuration T is S20
endothermic, with Ead = 0.52 eV. In the case of p-type GaSe, it is expected that chemisorption will be even weaker than what is predicted by this value. However, more detailed calculation of the activation barrier will be necessary to understand the favorable oxygen adsorption of GaS more clearly. Band structure analysis of GaS Configuration B shows that the valence band (VB) is rather flat, as similar as that shown in figure. The 1OB/(44 GaS) structure has an indirect gap (K ) of Eg = 2.40 eV, which is 0.18 eV smaller than the Eg = 2.58 eV of pristine GaS ((K,) M). The direct band gap is 2.45 eV at the point, making the difference between the direct and indirect band gaps only 0.05 eV, which is a value comparable to the thermal energy (3kT/2 = 0.039 eV) at room temperature. Charge density analysis reveals that the VB more or less represents localized p(S) and p(Ga) states around the electronegative oxygen atom, rather than being delocalized through the whole sheet. This may explain why the VB is appreciably flatter than that of pristine GaS. For the same reason, the CB also becomes more or less flat and more stabilized, which could be responsible for the decrease in the band gap. Also note that the M point of the first Brillouin zone in the 11 cell folds back to the point in the 44 cell, which means that oxygen adsorption causes the direct and indirect band gaps to converge. The 1OB/(66 GaS) structure has a direct band gap of 2.46 eV, which is 0.11 eV smaller than the band gap of pristine GaS. The direct gap is now only 0.004 eV above the indirect gap, indicating that GaS becomes practically a direct band gap material at a proper oxygen coverage. It is important to note here that even just a small oxygen coverage (i.e., O:(Ga,S) = 1:128) can have a drastic effect on the electronic structure of GaS, with oxygen atoms being directly adsorbed from the oxide surface of the substrate. Based on these calculations, we believe that the enhancement in the CL of the GaS monolayer can be ascribed to the oxygen adsorption.
S21
Figure S11. (a) X-ray photoelectron spectroscopy (XPS) survey spectra, fine-scan (b) Ga 3p1/2 and 3p3/2 peaks, and (c) S 2s and Se 3s peaks of GaS and GaSe multilayers and their monolayers. (a)
C1s
O1s
Se3p, S2p, Ga3s
Ga3d, S3s Se3d
Intensity (arb. units)
Se3s, S2s
Ga3p
GaSe Monolayers
O Auger peak
C Auger peak
GaSe Multilayers GaS Monolayers GaS Multilayers
500
(b)
400
300
200
100
(c)
105.6 Si 2p
229.5
GaSe Monolayers
105.2
Intensity (arb. units)
0
229.3 GaSe Multilayers 226.5
105.5 104.9
Si 2p
GaS Monolayers 226.0 GaS Multilayers
107 eV (Ga 3p1/2)
114
111
108
99.3 eV (Si2p)
104 eV (Ga 3p3/2)
105
102
232 eV (Se 3s)
99 234
Binding energy (eV)
232
230
228 S 2s
228
226
224
222
Binding energy (eV)
The fine-scan Ga 3p1/2 and 3p3/2 peaks for the GaS multilayers and monolayers are blueshifted by about 0.9 and 1.5 eV from those of the neutral Ga (104 and 107 eV). For the monolayers, the peak at 103 eV is assigned to Si 2p of the SiOx/Si substrate. The GaS monolayers show a higher blue shift (0.6 eV) relative to GaS multilayers, which can be correlated with the adsorbed oxygen. The GaSe multilayers and monolayers show a blue shift peak of 1.2 and 1.6 eV from those of the neutral Ga, with the GaSe monolayers experiencing a greater blue shift (0.4 eV) relative to GaSe multilayers The GaS monolayers shows a more S22
blue-shift than the GaSe monolayers, 0.6 vs. 0.4 eV, suggesting the stronger binding with the adsorbed (electronegative) oxygen. The fine-scan S 2s peak for GaS multilayers and monolayers are red-shifted by about 2.0 and 1.5 eV from those of the neutral S (228 eV). The GaS monolayers show less of red shift (0.5 eV) relative to GaS multilayers, which could be related with the bonding with the adsorbed oxygen. The GaSe multilayers and monolayers show a red shift of 2.7 and 2.5 eV from those of the neutral Se 3s peak (232 eV), with the GaSe monolayers also showing less of a red shift (0.2 eV) relative to GaSe multilayers. The fact that the blue shift of the monolayers is more significant in the case of GaS than GaSe indicates that GaS monolayers have a stronger affinity for oxygen adsorption. We can therefore conclude that oxygen binding is more significant in monolayers than multilayers, and that GaS monolayers bind more strongly with oxygen than GaSe monolayers.
References S1. Hayek, M.; Brafman, O.; Lieth, R. M. A. Splitting and Coupling of Lattice Modes in the Layer Compounds GaSe, GaS, and GaSexS1-x. Phys. Rev. B 1973, 8, 2772-2779. S2. Gasanly, N. M.; Goncharov, A. F.; Melnik, N. N.; Ragimov, A. S. Optical Phonons in GaS1-xSex Layer Mixed Crystals. phys. stat. sol. (b) 1983, 120, 137-147. S3. Late, D. J.; Liu, B.; Matte, H. S. S. R.; Rao, C. N. R.; Dravid, V. P. Rapid Characterization of Ultrathin Layers of Chalcogenides on SiO2/Si Substrates. Adv. Funct. Mater. 2012, 22, 1894-1905. S4. Lee, C.; Yan, H.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. Anomalous Lattice Vibrations of Single and Few-Layer MoS2. ACS Nano 2010, 4, 2695-2700. S5. Huheey, J. E., Keiter, E. A., Keiter, R. L., Inorganic Chemistry. 4th. Ed.; Harper Collins College Publisher: 1993; Table E.1, pp A-35. S6. Kong, F.; Hu, C. L.; Hu, T.; Zhou, Y.; Mao, J. G. Explorations of New Phases in the GaIII/InIII–MoVI–SeIV/TeIV–O Systems. Dalton Trans. 2009, 4962-4970.
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