optical properties and photocarrier dynamics of bi o se
TRANSCRIPT
Optical Properties and Photocarrier Dynamics of Bi2O2Se Monolayer and
Nanoplates
Shuangyan Liu,1, ∗ Congwei Tan,2, ∗ Dawei He,1
Yongsheng Wang,1, † Hailin Peng,2, ‡ and Hui Zhao3, §
1Key Laboratory of Luminescence and Optical Information,
Ministry of Education, Institute of Optoelectronic Technology,
Beijing Jiaotong University, Beijing 100044, China2Center for Nanochemistry, Beijing National Laboratory for Molecular Sciences (BNLMS),
College of Chemistry and Molecular Engineering,
Peking University, Beijing 100871, China3Department of Physics and Astronomy, The University of Kansas,
Lawrence, Kansas 66045, United States
(Dated: November 11, 2019)
Abstract
We report a comprehensive experimental study on optical properties and photocarrier dynamics in
Bi2O2Se monolayers and nanoplates. Large and uniform Bi2O2Se nanoplates with various thicknesses down
to the monolayer limit were fabricated. In nanoplates, a direct optical transition near 720 nm was identified
by optical transmission, photoluminescence, and transient absorption spectroscopic measurements and was
attributed to the transition between the valence and conduction bands in the Γ valley. Time-resolved dif-
ferential reflection measurements revealed ultrafast carrier thermalization and energy relaxation processes
and a photocarrier recombination lifetime of about 200 ps in nanoplates. Furthermore, by spatially resolv-
ing the differential reflection signal, we obtained a photocarrier diffusion coefficient of about 4.8 cm2 s−1,
corresponding to a mobility of about 180 cm2 V−1 s−1. A similar direct transition is also observed in mono-
layer Bi2O2Se, suggesting that the states in the Γ valley do not change significantly with the thickness. The
temporal dynamics of the excitons in the monolayer is quite different from the nanoplates, with a strong
saturation effect and fast exciton-exciton annihilation at high densities. Spatially and temporally resolved
measurements yielded an exciton diffusion coefficient of about 20 cm2 s−1.
Keywords: two-dimensional material; Bi2O2Se; photocarrier; ultrafast process; diffusion; transient absorption
1
I. INTRODUCTION
The discovery of graphene has created a fast-growing interest in two-dimensional (2D) mate-
rials, such as transition metal dichalcogenides (TMDs) and phosphorene1. These atomically thin
materials possess several unique features that make them attractive to both fundamental research
and applications2–6. They also provide a new route to fabricate multilayer structures by stacking
together several 2D materials7. As such, it is highly desirable to discover new 2D materials with
different properties to expand the material library.
Very recently, Bi2O2Se has emerged as a new type of layered two-dimensional (2D) material
with a unique structure and novel electronic transport properties. Bi2O2Se crystals are formed by
tetragonal (BiO)n layers separated by Se atomic sheets. Instead of the van der Waals interlayer
coupling that is commonly found in layered materials, in Bi2O2Se the layered are coupled by
electrostatic forces. Bulk crystals of Bi2O2Se and Bi2O2S can be used as novel thermoelectric8–12
and photovoltaic materials13,14 with good room-temperature charge mobilities15,16. Very recently,
ultrathin and large-sized Bi2O2Se nanoplates were synthesized by epitaxy17–20. These materials
show room- and low-temperature mobilities as high as 450 and 20,000 cm2V−1S−1, respectively17,
which could be attributed to the suppression of carrier scattering by layer separation of the carriers
and the electron donors21. They also possess thickness-tunable optical bandgaps22 and low dop-
ing concentrations19 - both are attractive features for electronic and optoelectronic applications.
Indeed, several types of devices based on ultrathin Bi2O2Se have been demonstrated, including
transistors20,23,24, infrared photodetectors18,23,25–27, optical switches28, and memristor29. From a
fundamental point of view, its non-van-der-Waals interlayer coupling offers a unique platform to
study effect of interlayer interaction on properties of layered structures.
The fast process on material synthesis and device demonstration invites studies of the physical
properties of Bi2O2Se, which will provide a foundation for future development of this exotic mate-
rial. For example, the high-quality samples have allowed measurement of the electronic structures
of this material30. However, the optical properties of Bi2O2Se, especially the dynamical properties
of photocarriers, have been less studies, despite their importance for most optoelectronic devices.
Here we report steady-state optical and spatiotemporally resolved pump-probe measurements on
optical properties and photocarrier dynamics in both monolayers and nanoplates of Bi2O2Se. We
obtained important parameters describing the carrier dynamics, such as the recombination life-
times and the diffusion coefficients. These results provide basic information that is useful for
2
understanding, designing, and optimizing electronic and optoelectronic devices based on ultrathin
Bi2O2Se.
II. RESULTS AND DISCUSSION
A. Bi2O2Se Nanoplates
Nanoplates of Bi2O2Se, with a lattice structure schematically shown in Figure 1(a), were grown
on mica substrates by chemical vapor deposition (CVD). Figure 1(b) shows an optical microscopic
image of several Bi2O2Se nanoplates with a squared shape and with sizes as large as 30× 30 µm2.
An atomic force microscopic image of a representative nanoplate is shown in Figure 1(c), which
yields a thickness of about 13 nm. This corresponds to 21 layers, according to the monolayer
thickness of about 0.61 nm17. We also measured the Raman spectrum of this nanoplate with a
532-nm laser beam, as shown in Figure 1(d). The pronounced peak at 160 cm−1 is assigned to the
A1g mode, which originates from the out-of-plane vibration of Bi atoms according to previously
reported analyses12,16,31.
Previous studies have shown that the lowest states in the conduction band and the highest states
in the valence band in Bi2O2Se nanoplates are located in the X and Γ valleys, respectively17,30.
Hence, Bi2O2Se nanoplates are indirect semiconductors. The bandgap is about 0.8 eV17,30. For
optical applications, however, it is desirable to utilize direct transitions that can provide large
absorption coefficients and efficient photoluminescence (PL). To probe direct optical transitions
in Bi2O2Se nanoplates, we performed PL, transmission, and differential reflection spectroscopic
measurements. First, we obtained the PL spectrum of the sample, as shown as the red curve in Fig-
ure 2(a), under the excitation of a 532-nm continuous-wave laser. The PL peak is at about 720 nm
(1.72 eV), with a width of about 70 nm. This width is about twice larger than that of typical TMD
monolayers. We attribute this width to the inhomogeneous broadening due to defects in addition
to the homogeneous broaden during to phonons. Second, the transmission spectrum, Figure 2(b),
shows a reduction of transmission, and thus an increase of absorption, in the range of 680 - 800
nm. This absorption feature is reasonably consistent with the PL line shape. Finally, we measured
the differential reflection spectrum. A 410-nm pump pulse was used to inject photocarriers in the
Bi2O2Se nanoplate. The differential reflection of a probe pulse, defined as ∆R/R0 = (R − R0)/R0,
where R and R0 are the probe reflection with and without the presence of the pump, respectively,
3
was measured by a homemade transient absorption microscope (see Experimental Section)32 . The
peak differential reflection signal, obtained by arranging the probe pulse to arrive at the sample at
about 0.5 ps after the pump pulse, was measured as the probe wavelength was tuned. The results
are plotted in Figure 2(a) as the blue symbols. Despite of the small range of the wavelengths used
due to the limited tuning range of the instrument in this configuration, we observed a wavelength
dependence that agrees well with the PL line shape. We note that the spectrum of the differen-
tial reflection can be treated as that of transient absorption only for extremely thin films, and a
full Fresnel analysis would be required to accurately correlate the two spectra from this 13-nm
sample.33. However, several parameters needed for such an analysis are unknown for this new
material. Nevertheless, the results presented in Figure 2 establish a direct optical transition at
about 1.7 eV. By comparing with the electronic structure from first-principles calculations17, we
can assign it to the transition between the conduction-band and valence-band states in the Γ valley.
We also note that this transition was not observed in previously fabricated samples, which could
be attributed to their higher doping concentrations22.
We next used this direct transition to study photocarrier dynamics in nanoplates. Several
nanoplates were studied and similar results were obtained. Here, we focus on the 13-nm nanoplate
shown in Figure 1. Using a 720-nm probe, we measured time-dependent differential reflection
signal with various energy fluences of the 410-nm pump pulse. As shown in Figure 3(a), the sig-
nal reaches a peak shortly after the pump excitation. The rising part of the signal can be fit by
the integral of a Gaussian function with a full width at half maximum of 0.6 ps, as indicated by
the red curve. This time is slightly longer than the instrument response time of 0.4 ps. Since the
photocarriers are injected with large excess energies by the 410-nm pump pulse and are detected
by the probe that is tuned to the direct bandgap, the rising of the signal could be attributed to the
thermalization and energy relaxation of hot carriers. The peak signal is proportional to the pump
fluence, as shown in Figure 3(c). Since the injected carrier density is proportional to the pump
fluence, this result establishes the linear relation between the differential reflection signal and the
photocarrier density. We note that the highest fluence used in this study, 32.7 µJ cm−2, corresponds
to an injected areal carrier density of about 1.65 ×1012 cm−2 in the first layer of Bi2O2Se (see Ex-
perimental Section) and an average distance of 8 nm between two pairs of photocarriers. After
the peak, the signal decays exponentially. The yellow curves in Figure 3(b) are single exponential
fits. The decay time constants obtained from these fits are plotted in Figure 3(d). The weak depen-
dence of the decay constant on the pump fluence suggests that the carrier-carrier interaction has a
4
200 300 400 500 600 700
Inte
nsity
(a.u
.)
Raman Shift (cm-1)
(b)(a)
(c) (d)
0 2 4 6 8 1016
18
20
22
24
26
28
30
32
34
Hei
ght (
nm)
Distance (m)
13 nm
FIG. 1. (a) Crystalline structure of Bi2O2Se. The purple, golden, and red spheres represent Bi, O, and Se
atoms, respectively. (b) Optical microscopic image of Bi2O2Se nanoplates on a mica substrate. (c) Atomic
force microscopic image and height profile [inset of (d)] of the large nanoplate shown in (b). The thickness
of 13 nm corresponds to 21 layers. (d) Raman spectrum of the nanoplate shown in (b) measured with a
532-nm laser beam.
minor effect on the recombination dynamics. Hence, we conclude that the recombination lifetime
of the photocarriers in Bi2O2Se nanoplates is about 200 ps. This lifetime is longer than that of 2D
TMDs32.
To study the transport properties of photocarriers in Bi2O2Se nanoplates, we performed spa-
tially and temporally resolved pump-probe measurements with pump and probe laser spot sizes of
about 2 µm. The probe spot was scanned across the pump spot by tilting a mirror in the probe arm
and thus changing the incident angle of the probe to the objective lens. At each probe position,
the differential reflection signal was measured as a function of the probe delay. Figure 4(a) shows
the differential reflection signal as a function of the probe delay and the probe position, which is
defined as the position of the probe spot center with respect to the pump spot center. To analyze
5
640 660 680 700 720 740 760 780 800 8200
20
40
60
0.00.51.01.52.02.53.0
Pea
k R
/R0 (
10-3
) 0
50
100
150
PL (c
ount
s pe
r sec
ond)
T (%
)
Wavelength (nm)
(a)
(b)
FIG. 2. (a) Peak differential reflection of the 13-nm nanoplate as a function of the probe wavelength (blue
symbols, left axis) measured with a 410-nm pump and the photoluminescence spectrum of the same sam-
ple (red curve, right axis) under the 532-nm laser excitation. (b) The transmission spectrum of the same
nanoplate.
the spatiotemporal dynamics, we plot the spatial profiles at all the probe delays and fit them with
Gaussian functions. Figure 4(b) shows a few examples of the profiles and the fits. The obtained
widths of the profiles, defined as the full widths at half maximum, are squared and plotted as a
function of the probe delay in Figure 4(c). We note that the probe laser spot size does not change
with the delay, as confirmed by using the imaging system. Furthermore, since the probe delay was
scanned at each probe position, any drift of the spot during the measurement would have resulted
in asymmetric spatial profiles, instead of an artifact of broadening.
The expansion of the profile observed in Figure 4(c) is clear evidence of in-plane diffusion of
photocarriers. The tightly focused pump spot injects photocarriers with a narrow spatial distribu-
tion. The density gradient drives the diffusion of the photocarriers away from the center of the
distribution. By solving the diffusion-recombination equation, it is straightforward to show that,
for an initial Gaussian distribution, the profile remains Gaussian during the whole process with the
width increasing as34
w2(t) = w20 + 16ln(2)Dt, (1)
where w0 is the initial width, which is determined by the convolution of the pump and probe spots,
and D, the diffusion coefficient. The linear increase of the squared width is consistent with the
6
-1 0 1 2 3 4
0
1
2
3
4
5
R
/R0 (
10-3)
Probe Delay (ps)0 200 400 600 800
0
1
2
3
4
5
Pump Fluence(J cm-2)
0.8 2.0 3.3 4.9 8.210.212.316.320.424.532.7
R
/R0 (
10-3)
Probe Delay (ps)
0 5 10 15 20 25 30 35
0
1
2
3
4
5
6
Peak
R
/R0 (
10-3)
Pump Fluence (J cm-2)0 5 10 15 20 25 30 35
0
50
100
150
200
250
Dec
ay T
ime
(ps)
Pump Fluence (J cm-2)
(a) (b)
(c) (d)
FIG. 3. (a) Differential reflection signal of the 13-nm nanoplate as a function of the probe delay measured
with a 720-nm probe and 410-nm pump pulses with various fluences. (b) The same as (a) but with a larger
range of probe delays and with exponential fits (curves). (c) Peak differential reflection signal as a function
of the pump fluence. The red line is a linear fit. (d) The decay time constant deduced from the fits shown in
(b) as a function of the pump fluence.
data shown in Figure 4(c) . A linear fit, shown as the red line, gives a diffusion coefficient of
4.8 ± 0.5 cm2 s−1. The slight deviation from the linear expansion at later probe delays could be
attributed to the effect of defects on photocarrier transport. From the diffusion coefficient and
the carrier lifetime of τ = 200 ps, we can deduce a diffusion length of L =√
Dτ = 310 nm.
Furthermore, by using the Einstein’s relation, D/kBT = µ/e, where kB, e, and T = 300 K are the
Boltzmann constant, the elementary charge, and the carrier temperature, respectively, we obtained
a photocarrier mobility of about 180 cm2 V−1 s−1.
In general, photocarriers in semiconductors can exist in the forms of free electron-hole pair
and exciton - the quasiparticle formed by an electron and a hole tightly bound by their Coulomb
attraction. For a thermalized carrier system, the distribution between these two forms is determined
7
-4 -2 0 2 40
1
2
3
4
5
Probe Delay 1 ps21 ps41 ps62 ps
R
/R0
(10-3
)
Probe Position (m)-4 -2 0 2 4
60
50
40
30
20
10
0
Probe Position (m)
Prob
e D
elay
(ps)
-0.1
5.3
R/R0 (10-3)
0 10 20 30 40 50 606.1
6.2
6.3
6.4
6.5
w2 (
m2 )
Probe Delay (ps)
D = 4.8 0.5 cm2 s-1
(a) (b) (c)
FIG. 4. (a) Differential reflection signal of the 13-nm Bi2O2Se nanoplate as a function of both the probe
delay and the probe position. (b) Examples of the spatial profiles of the differential reflection signal at probe
delays as labeled in the figure. The red curves are Gaussian fits. (c) The squared width of the spatial profiles
obtained by Gaussian fits as a function of the probe delay. The linear fit, shown as the red line, gives a
diffusion coefficient of about 4.8 cm2 s−1.
by the ratio between the exciton binding energy and the thermal energy of the lattice. So far, there
have been no reports on the exciton binding energy or excitonic effects in Bi2O2Se nanoplates.
Hence, it is unclear whether the observed dynamics is dominated by the excitons or the electron-
hole pairs. If the excitonic effect is not strong in this material, the measured diffusion coefficient
describes the ambipolar diffusion of the free electron-hole pairs.
B. Bi2O2Se Monolayer
We studied monolayer Bi2O2Se samples using the similar experimental approaches. A Bi2O2Se
monolayer on a mica substrate is shown in Figure 5(a). Unlike the nanoplates that all have regular
squared shapes, the monolayer flake has an irregular shape. However, the flake is very large,
about 100 µm in length, and uniform. Atomic force microscopic measurements, shown in Figure
5(b), resulted in a thickness of 0.82 nm, confirming its monolayer nature18,20. Figure 5(c) shows
a Raman spectrum of the monolayer sample. According to theory, the Raman active modes in
monolayer Bi2O2Se include two double-degenerated Eg modes and two non-degenerated A1g and
B1g modes35. We observed five distinct Raman peaks in Figure 5(c). The peak at 160 cm−1 is
assigned to the active mode of A1g, similar to the nanoplate shown in Figure 1. The peak at 312
cm−1 could be attributed to the B1g-like mode due to the out of-plane vibration of the O atoms.
8
100 200 300 400 500 600 700 800
Inte
nsity
(a.u
.)
Raman Shift (cm-1)660 680 700 720 740 760 780 8000
50
100
150
200
Wavelength (nm)
PL (c
ount
s pe
r sec
ond)
0
2
4
6
Peak
R
/R0 (
10-4)
(c) (d)
(b)(b)
0 1 2 3 4 5 6-0.4
-0.2
0.0
0.2
0.4
0.6
Hei
ght (
nm)
Distance (m)
0.82 nm
FIG. 5. (a) Optical microscopic image of a Bi2O2Se monolayer on a mica substrate. (b) Atomic force
microscopic image and height profile [inset in (d)] showing a thickness of 0.82 nm, corresponding to a
monolayer. (c) Raman spectrum of the monolayer measured with a 532-nm laser beam. (d) Peak differential
reflection of the monolayer as a function of the probe wavelength (blue symbols, right axis) measured with
a 410-nm pump pulse and the photoluminescence spectrum (red curve, left axis) under a 532-nm laser
excitation.
However, there is a discrepancy of 42 cm−1 with the theoretical value (354.3 cm−1)35. We note that
so far, B1g-like mode of this material has not been reported. Besides, the peak at 514 cm−1 could be
assigned to the Eg-like mode, although the wavenumber is higher than the theoretical value (433.3
cm−1)35.
To identify a direct optical transition in monolayer Bi2O2Se, we performed PL measurements
with a 532-nm excitation laser. The PL spectrum, shown as the red curve in Figure 5(d), is similar
to the 13-nm nanoplate, with a similar width and a small blue shift of only a few nanometers. This
observation suggests that the electronic bandstructure in the Γ valley is not strongly affected by
the thickness. The blue symbols in Figure 5(d) show the peak differential reflection signal as a
function of the probe wavelength, measured with a 410-nm pump. The result also shows a peak at
9
about 720 nm. Due to the small absorbance of the monolayer compared to the thicker nanoplates,
we could not obtain a reliable transmission spectrum. Nevertheless, the spectra shown in Figure 5
establish a direct transition at about 720 nm, which is similar to the nanoplates.
0 100 200 300 400-1
0
1
2
3
4
5
6
7
8
R
/R0
(10-4
)
Probe Delay (ps)0 2 4 6 8 10 12
-1
0
1
2
3
4
5
6
7
8
Pump Fluence (J cm-2) 0.9 1.8 3.2 5.5
R
/R0
(10-4
)
Probe Delay (ps)1 2 3 4 5 6
2
3
4
5
6
7
Peak
R
/R0
(10-4
)
Pump Fluence (J cm-2)
(a) (b) (c)
FIG. 6. (a) Differential reflection signal of the Bi2O2Se monolayer as a function of the probe delay measured
with a 716-nm probe and a 410-nm pump with various pump fluences as labeled. (b) The same as (a) but
with a larger range of delays. (c) Peak differential reflection signal as a function of the pump fluence. The
red line is a fit with a saturation fluence of 5.6 µJ cm−2 (see text).
We next used a 716-nm probe and a 410-nm pump to study photocarrier dynamics in monolayer
Bi2O2Se. Figure 6(a) and 6(b) shows the differential reflection signal (over a short and long time
ranges, respectively) measured with different values of the pump fluence. One distinct difference
from the nanoplate results is that, there is a strong fluence dependence of the dynamics. At rela-
tively high fluences, such as 5.5 µJ cm−2 (corresponding to an areal carrier density of 2.7 × 1011
cm−2 or an average photocarrier distance of 19 nm), there is a fast-decay component of the dynam-
ics, which is absent when the density is low, as shown in Figure 6(a). Furthermore, the peak signal
is a nonlinear function of the pump fluence, with a significant saturation effect, as shown in Figure
6(c). The behavior could be described by a saturation model36 that is widely used for nonlinear
absorption, ∆R/R0 ∝ F/(F + Fsat). By a fit, shown as the red curve in Figure 6(c), we obtained a
saturation fluence of Fsat = 5.6 ± 0.6 µJ cm−2. This corresponds to a saturation density of about
2.8 × 1011 cm−2. This value is comparable to those obtained from other 2D semiconductors, such
as WS237,38. This effect can be attributed to the phase-space filling effect of the photocarriers, and
could be utilized in developing saturable absorbers. At lower densities, the fast decay channel is
absent and the decay can be described as a density-independent exponential decay process. The
orange curve in Figure 6(b) shows an example of single-exponential fits to the data in this regime,
producing a decay time constant of about 100±20 ps. This constant is assigned as the photocarrier
10
lifetime in monolayer Bi2O2Se.
It has been well established through recent studies that in 2D semiconductors, the exciton bind-
ing energies are orders-of-magnitude larger than their 3D counterparts due to the reduced dielec-
tric screening. For example, in 2D TMD semiconductors, excitons are formed from photo-excited
electron-hole pairs on a sub-picosecond time scale39,40 and are stable at room temperature4,41. The
dominance of excitons are considered a general feature of 2D semiconductors, since the reduction
of the dielectric screening is a geometric, rather than a material, effect. Hence, although there
have been no reports on excitonic effects in monolayer Bi2O2Se, it is safe to expect that at least in
this 2D limit, excitons are the dominant form of photocarriers. Thus, the behaviors summarized in
Figure 6 reflects excitonic dynamics in monolayer Bi2O2Se.
To further understand the dynamics in the high-density regime, we include a bi-molecular
exciton-exciton annihilation process. This process has been generally observed in systems
with strong interactions between excitons, such as organic crystals42–44, semiconducting carbon
nanotubes45,46, and 2D semiconductors47–51. With this process, the rate equation of the exciton
density can be written asdNdt
= −1τ
N −12γN2, (2)
where τ and γ are the exciton recombination lifetime and the exciton-exciton annihilation rate,
respectively. If the first item on the right-hand side of Eq. 2 is much smaller than the second item,
we can ignore it and find a simple solution of
N0
N(t)− 1 = γN0t, (3)
where N0 is the initially injected exciton density. This approximation is valid when γNτ/2 � 1.
To better compare the experimental results with this model, we first converted the measured
differential reflection signal to the exciton density by using the nonlinear relation represented as
the red curve in Figure 6(c). A few examples are shown in Figure 7(a). Then, the time evolution of
the quantities N0/N−1 was calculated from these curves and plotted in Figure 7(b). At early times,
this quantity indeed increases linearly, validating the approximation of ignoring the first item on
the right-hand side of Eq. 2. By linear fits, shown as the red lines, we deduce the slope, γN0, for
each value of N0, and plot it as a function of N0 in (c). By a linear fit according to the model, we
obtained an exciton-exciton annihilation rate of γ of about 0.21 cm2 s−1. Interestingly, this value
is similar to previously reported exciton-exciton annihilation rate of other 2D semiconductors,
such as MoSe247, MoS2
49, and WS251, suggesting that the enhanced exciton-exciton interaction
11
(a) (b) (c)
048
12
048
04
04
0 20 40 60 80 100 120
0
4
0
4
N0/N
-1
Probe Delay (ps)0.0 0.5 1.0 1.5 2.0 2.5
0
1
2
3
4
5
6
7
8
Slop
e (1
010 s
-1)
Injected Carrier Density (1011 cm-2)
0123
012
012
012
01
0 50 100 150 200 250 300 350 40001
5.5
Pump Fluence (J cm-2)
Probe Delay (ps)
4.5
4.1
3.6
3.2
N (1
011 c
m-2)
2.7
FIG. 7. (a) Time evolution of the exciton density with various pump fluences calculated from the time-
dependent differential reflection signals by using the relation shown as the red curve in Figure 6(c). (b)
Time evolution of the quantity, N0/N − 1, calculated from (a). (c) The slope obtained from linear fits shown
as the red lines in (b) as a function of N0. The red line is a linear fit.
in such 2D structures could be due to the geometric effect. We note that the bimolecular carrier
recombination was also observed in bulk Bi2O2Se crystals with very high carrier densities (two
orders of magnitude higher than used in this study).14.
-4 -2 0 2 480
70
60
50
40
30
20
10
0
Probe Position (m)
Prob
e D
elay
(ps)
0.0
10.9R/R0 (10-4)
-6 -4 -2 0 2 4 6
0
2
4
6
8
10
12
Probe Delay
1 ps 21 ps 41 ps 84 ps
Probe Position (m)
R
/R0 (
10-4)
0 20 40 60 80
10
11
12
13
w2 (
m2 )
Probe Delay (ps)
Equation y = a + b*xWeight InstrumentalResidual Sum of Squares
24.95656
Pearson's r 0.58727Adj. R-Square 0.28533
Value Standard Error
?$OP:A=1Intercept 10.49421 0.58446Slope 0.02444 0.01016
D = 20 10 cm2 s-1
(a) (b) (c)
FIG. 8. (a) Differential reflection signal of the monolayer Bi2O2Se sample as a function of both the probe
delay and the probe position. (b) Examples of the spatial profiles of the differential reflection signal at
several probe delays as labeled in the figure. The red curves are Gaussian fits. (c) The squared width of the
spatial profiles obtained by Gaussian fits as a function of the probe delay. The linear fit shown as the red
line gives a diffusion coefficient of about 20 cm2 s−1.
Finally, we performed spatially and temporally resolved differential reflection measurements
to study the exciton transport properties in monolayer Bi2O2Se. The results are summarized in
Figure 8. The procedure of the measurement is the same as that used for the nanoplates. However,
slightly larger laser spots of 2.6 µm were used in order to inject a lower carrier density while
12
maintaining the same optical power at the detector. The differential reflection signal as a function
of the probe delay and position is shown in Figure 8(a). Figure 8(b) shows a few examples of
the spatial profiles of the signal and the corresponding Gaussian fits. The widths of the profiles
deduced from the fits are plotted in Figure 8(c). The larger uncertainties in the width, compared to
the nanoplate measurement, are due to the lower signal and thus poor signal-to-noise ratio. Unlike
the nanoplate measurement where the signal is proportional to the carrier density and the carrier
recombination is independent of the density, here the nonlinear relation between the signal and
the exciton density as well as the exciton-exciton annihilation complicates the interpretation of the
results. For example, the increase of the width at early probe delays could have a contribution from
the faster decay of the exciton density near the spot center, where the exciton density is higher.
However, at later probe delays, the density has dropped to low values and these factors become
less important. By fitting the widths after 40 ps, we deduced an exciton diffusion coefficient of
about 20 ± 10 cm2 s−1. This value is a few times large than the nanoplates, suggesting better
transport performance of monolayer Bi2O2Se. With the same procedures used in analyzing the
nanoplate, we obtained a diffusion length of 450 nm and an exciton mobility of about 770 cm2 V−1
s−1. This value is comparable to the room-temperature charge carrier mobilities in 2D Bi2O2Se
obtained from transport measurements17,27,52 in the range of 300 - 450 cm2 V−1 s−1 . However, it
should be noted that the exciton mobility and the charge carrier mobilities are different physical
quantities that are related to various scattering mechanisms differently53.
III. CONCLUSION
We have presented a comprehensive experimental study on optical properties and photocarrier
dynamics in the newly emerged 2D semiconductor, Bi2O2Se. Large scale and uniform samples of
Bi2O2Se, including nanoplates and monolayers, were fabricated by epitaxy. For the nanoplates,
transmission, photoluminescence, and transient absorption spectroscopic measurements allowed
identification a direction optical transition near 720 nm, which is assigned to the transition in the Γ
valley. Time-resolved differential reflection measurements revealed ultrafast carrier thermalization
and energy relaxation processes, and a 200-ps photocarrier recombination lifetime. Furthermore,
by measuring the spatiotemporal dynamics of the photocarriers, we deduced a photocarrier diffu-
sion coefficient of about 4.8 cm2 s−1. For the monolayer Bi2O2Se, a similar direct transition was
observed, suggesting that the direct transition in the Γ valley does not depend strongly on the thick-
13
ness. The temporal dynamics of the excitons in monolayers shows a strong saturation effect with
a saturation density of about 2.8 ×1011 cm−2 and significant exciton-exciton annihilation effect.
Spatially and temporally resolved measurements revealed an exciton diffusion coefficient as high
as 20 cm2 s−1. The differences in the carrier dynamics in monolayer and nanoplate samples could
be attributed to their different electronic structures and the reduced dielectric screening in mono-
layers. However, further theoretical studies are necessary to fully understand such differences.
The results of this study provide basic information about the optical properties and, in particular,
dynamical properties of photocarriers in Bi2O2Se nanoplates and monolayers. The information is
useful for understanding electronic and optical properties of this new material, evaluating its suit-
ability for various optoelectronic applications, and designing optoelectronic devices with optimal
performances. For example, the strong optical absorption saturation and exciton-exciton interac-
tion in monolayer Bi2O2Se make it a promising nonlinear optical material. Its direction optical
transition in near infrared and good excitonic transport performance are also attractive for infrared
photodetection applications.
IV. EXPERIMENTAL SECTION
A. CVD Synthesis of Bi2O2Se Nanoplates on Mica
The 2D Bi2O2Se nanoplates were synthesized by using a home-made CVD system. In detail,
the tube furnace is equipped with a 12-inch-long and 30-mm-diameter quartz tube. Bi2O3 powders
(Alfa Aesar, 99.995 %) and Bi2Se3 bulks (Alfa Aesar, 99.995 %) were adopted as precursors and
placed in two quartz boat. Bi2O3 and Bi2Se3 were located in the central zone and in the upstream
with a distance of 6 cm, respectively. Then, the freshly cleaved mica substrates were placed
downstream from the hot center with a distance of 9 to 12 cm. The typical flow rate of 200 sccm
Ar was introduced as the carrier gas to keep the chamber pressure of 400 Torr with the assistance
of metal value. The temperature of central zone was heated to 615 ◦C and held for 20 min. After
that, the furnace was cooled down naturally to room temperature.
B. Steady-State Optical Measurements
Absorption spectroscopy was performed with a homemade setup. A broadband and incoherent
light beam is focused to a spot size of about 2 - 3 µm by an objective lens. The transmitted
14
or reflected light from the sample is collimated and directed to a spectrometer equipped with a
thermoelectrically cooled charge-coupled-device camera. Absorption spectra of the samples are
obtained by comparing the spectra acquired from the sample and from the substrate, respectively,
under the same conditions. The PL spectroscopy was carried out with the same setup but with a
532-nm continuous-wave laser as the excitation source.
C. Transient Absorption
Photocarrier dynamics was studied by a homemade transient absorption setup with high spatial
and temporal resolution32. An 820-nm and 100-fs pulse from a 80-MHz Ti:sapphire oscillator
is divided into two parts by a beamsplitter. One part pumps an optical parametric oscillator to
generate a visible output, serving as the probe pulse. The other part is focused to a beta barium
borate crystal to generate its second harmonic at 410 nm, used as the pump pulse. The pump
and probe pulses are combined by a beamsplitter and focused onto the sample by a microscope
objective lens. The reflected probe is collimated by the same objective lens and sent to a silicon
photodiode, which outputs a voltage signal that is proportional to the average power of the probe
reflected by the sample and received by the photodiode. A mechanical chopper is install in the
pump arm to modulate the pump power reaching the sample at about 2 KHz. Thus, the output of
the photodiode alternates between two voltages corresponding to the probe reflections with and
without the presence of the pump, respectively. By using a lock-in amplifier that is referenced
to the modulation frequency, we can measure the differential reflection, ∆R/R0. To reveal the
spatiotemporal dynamics of the photocarriers, the differential reflection is measured as a function
of the probe delay (i.e. the time lag of the probe pulse with respect to the pump pulse) by changing
the distance that the probe pulse propagates with a linear stage, and as a function of the probe spot
position with respect to the pump spot by changing the incident angle of the probe beam to the
objective lens, which is achieved by tilting a mirror in the probe arm. During the measurements,
the samples were kept at room temperature and under ambient condition.
To estimate the injected carrier density for a certain value of the pulse fluence of the 410-nm
pump, we measured the absorbance (A) of the t = 13 nm nanoplate for a normal incident 410-nm
pulse. This was achieved by comparing the reflected and transmitted powers with respect to the
incident power. We found a value of about 0.17, which corresponds to an absorption coefficient of
α = 2.303 A/t of 3 × 107 m−1. With this value, we found that a pump pulse with a peak fluence of
15
1 µJ cm−2 injects a peak volume carrier density of about 6.2 × 1017 cm−3 at the center of the spot
and at the front surface of the sample. This corresponds to an areal carrier density of 5.0 × 1010
cm−2 in the first layer of Bi2O2Se. Assuming the same absorption coefficient at 410 nm for the
monolayer, the fluence of 1 µJ cm−2 corresponds to an injected areal carrier density of the same
value.
V. ACKNOWLEDGMENTS
We are grateful for the financial support of the National Key R&D Program of China (2016
YFA0202302), the National Natural Science Foundation of China (61527817, 61875236, 61905010,
61905007), and National Science Foundation of USA (DMR-1505852).
∗ These two authors contributed equally
1 Z. Lin, A. McCreary, N. Briggs, S. Subramanian, K. H. Zhang, Y. F. Sun, X. F. Li, N. J. Borys, H. T.
Yuan, S. K. Fullerton-Shirey, A. Chernikov, H. Zhao, S. McDonnell, A. M. Lindenberg, K. Xiao, B. J.
LeRoy, M. Drndic, J. C. M. Hwang, J. Park, M. Chhowalla, R. E. Schaak, A. Javey, M. C. Hersam,
J. Robinson, and M. Terrones, 2D Mater. 3, 042001 (2016).
2 A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli, and F. Wang, Nano Lett. 10, 1271
(2010).
3 K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, Phys. Rev. Lett. 105, 136805 (2010).
4 K. He, N. Kumar, L. Zhao, Z. Wang, K. F. Mak, H. Zhao, and J. Shan, Phys. Rev. Lett. 113, 026803
(2014).
5 N. Kumar, S. Najmaei, Q. Cui, F. Ceballos, P. M. Ajayan, J. Lou, and H. Zhao, Phys. Rev. B 87, 161403
(2013).
6 A. Autere, C. R. Ryder, A. Saynatjoki, L. Karvonen, B. Amirsolaimani, R. A. Norwood, N. Peygham-
barian, K. Kieu, H. Lipsanen, M. C. Hersam, and Z. P. Sun, J. Phys. Chem. Lett. 8, 1343 (2017).
7 A. K. Geim and I. V. Grigorieva, Nature 499, 419 (2013).
16
8 B. Zhan, Y. C. Liu, X. Tan, J. L. Lan, Y. H. Lin, and C. W. Nan, J. Am. Ceramic Soc. 98, 2465 (2015).
9 T. V. Quang and M. Kim, J. Appl. Phys. 120, 195105 (2016).
10 X. Tan, J. L. Lan, G. K. Ren, Y. C. Liu, Y. H. Lin, and C. W. Nan, J. Am. Ceramic Soc. 100, 1494
(2017).
11 J. B. Yu and Q. Sun, Appl. Phys. Lett. 112, 053901 (2018).
12 T. Cheng, C. W. Tan, S. Q. Zhang, T. Tu, H. L. Peng, and Z. R. Liu, J. Phys. Chem. C 122, 19970
(2018).
13 X. W. Zhang, B. Wang, X. H. Niu, Y. H. Li, Y. F. Chen, and J. L. Wang, Mater. Horizons 5, 1058 (2018).
14 C. H. Zhu, T. Tong, Y. J. Liu, Y. F. Meng, Z. H. Nie, X. F. Wang, Y. B. Xu, Y. Shi, R. Zhang, and F. Q.
Wang, Appl. Phys. Lett. 113, 061104 (2018).
15 C. Drasar, P. Ruleova, L. Benes, and P. Lostak, J. Electro. Mater. 41, 2317 (2012).
16 T. Tong, M. H. Zhang, Y. Q. Chen, Y. Li, L. M. Chen, J. R. Zhang, F. Q. Song, X. F. Wang, W. Q. Zou,
Y. B. Xu, and R. Zhang, Appl. Phys. Lett. 113, 072106 (2018).
17 J. X. Wu, H. T. Yuan, M. M. Meng, C. Chen, Y. Sun, Z. Y. Chen, W. H. Dang, C. W. Tan, Y. J. Liu, J. B.
Yin, Y. B. Zhou, S. Y. Huang, H. Q. Xu, Y. Cui, H. Y. Hwang, Z. F. Liu, Y. L. Chen, B. H. Yan, and
H. L. Peng, Nat. Nanotechnol. 12, 530 (2017).
18 U. Khan, Y. T. Luo, L. Tang, C. J. Teng, J. M. Liu, B. L. Liu, and H. M. Cheng, Adv. Funct. Mater. 29,
1807979 (2019).
19 J. X. Wu, C. G. Qiu, H. X. Fu, S. L. Chen, C. C. Zhang, Z. P. Dou, C. W. Tan, T. Tu, T. R. Li, Y. C.
Zhang, Z. Y. Zhang, L. M. Peng, P. Gao, B. H. Yan, and H. L. Peng, Nano Lett. 19, 197 (2019).
20 C. W. Tan, M. Tang, J. X. Wu, Y. N. Liu, T. R. Li, Y. Liang, B. Deng, Z. J. Tan, T. Tu, Y. C. Zhang,
C. Liu, J. H. Chen, Y. Wang, and H. L. Peng, Nano Letters 19, 2148 (2019).
21 H. X. Fu, J. X. Wu, H. L. Peng, and B. H. Yan, Phys. Rev. B 95, 241203 (2018).
22 J. X. Wu, C. W. Tan, Z. J. Tan, Y. J. Liu, J. B. Yin, W. H. Dang, M. Z. Wang, and H. L. Peng, Nano Lett.
17, 3021 (2017).
23 J. X. Wu, Y. J. Liu, Z. J. Tan, C. W. Tan, J. B. Yin, T. R. Li, T. Tu, and H. L. Peng, Adv. Mater. 29,
1704060 (2017).
24 R. G. Quhe, J. C. Liu, J. X. Wu, J. Yang, Y. Y. Wang, Q. H. Li, T. R. Li, Y. Guo, J. B. Yang, H. L. Peng,
M. Lei, and J. Lu, Nanoscale 11, 532 (2019).
25 J. Li, Z. X. Wang, Y. Wen, J. W. Chu, L. Yin, R. Q. Cheng, L. Lei, P. He, C. Jiang, L. P. Feng, and J. He,
Adv. Funct. Mater. 28, 1706437 (2018).
17
26 Q. D. Fu, C. Zhu, X. X. Zhao, X. L. Wang, A. Chaturvedi, C. Zhu, X. W. Wang, Q. S. Zeng, J. D. Zhou,
F. C. Liu, B. K. Tay, H. Zhang, S. J. Pennycook, and Z. Liu, Adv. Mater. 31, 1804945 (2019).
27 J. B. Yin, Z. J. Tan, H. Hong, J. X. Wu, H. T. Yuan, Y. J. Liu, C. Chen, C. W. Tan, F. R. Yao, T. R. Li,
Y. L. Chen, Z. F. Liu, K. H. Liu, and H. L. Peng, Nat. Commun. 9, 3311 (2018).
28 X. L. Tian, H. Y. Luo, R. F. Wei, C. H. Zhu, Q. Y. Guo, D. D. Yang, F. Q. Wang, J. F. Li, and J. R. Qiu,
Adv. Mater. 30, 1801021 (2018).
29 Z. Y. Zhang, T. R. Li, Y. J. Wu, Y. J. Jia, C. W. Tan, X. T. Xu, G. R. Wang, J. Lv, W. Zhang, Y. H. He,
J. Pei, C. Ma, G. Q. Li, H. Z. Xu, L. P. Shi, H. L. Peng, and H. L. Li, Adv. Mater. 31, 1805769 (2019).
30 C. Chen, M. X. Wang, J. X. Wu, H. X. Fu, H. F. Yang, Z. Tian, T. Tu, H. Peng, Y. Sun, X. Xu, J. Jiang,
N. B. M. Schroter, Y. W. Li, D. Pei, S. Liu, S. A. Ekahana, H. T. Yuan, J. M. Xue, G. Li, J. F. Jia, Z. K.
Liu, B. H. Yan, H. L. Peng, and Y. L. Chen, Sci. Adv. 4, eaat8355 (2018).
31 A. L. J. Pereira, D. Santamaria-Perez, J. Ruiz-Fuertes, F. J. Manjon, V. P. Cuenca-Gotor, R. Vilaplana,
O. Gomis, C. Popescu, A. Munoz, P. Rodriguez-Hernandez, A. Segura, L. Gracia, A. Beltran, P. Ruleova,
C. Drasar, and J. A. Sans, J. Phys. Chem. C 122, 8853 (2018).
32 F. Ceballos and H. Zhao, Adv. Funct. Mater. 27, 1604509 (2017).
33 D. J. Morrow, D. D. Kohler, Y. Zhao, S. Jin, and J. C. Wright, (2019), arXiv:1909.06445
[physics.optics].
34 D. A. Neamen, Semiconductor Physics and Devices (McGraw-Hill, Boston, 2002).
35 Y.-D. Xu, C. Wang, Y.-Y. Lv, Y. B. Chen, S.-H. Yao, and J. Zhou, RSC Adv. 9, 18042 (2019).
36 R. W. Boyd, Nonlinear Optics, 3rd ed. (Academy Press, San Diego, USA, 2008).
37 Q. Cui, Y. Li, J. Chang, H. Zhao, and C. Xu, Laser Photon. Rev. 13, 1800225 (2019).
38 S. Q. Zhao, D. W. He, J. Q. He, X. W. Zhang, L. X. Yi, Y. S. Wang, and H. Zhao, Nanoscale 10, 9538
(2018).
39 F. Ceballos, Q. Cui, M. Z. Bellus, and H. Zhao, Nanoscale 8, 11681 (2016).
40 P. Steinleitner, P. Merkl, P. Nagler, J. Mornhinweg, C. Schuller, T. Korn, A. Chernikov, and R. Huber,
Nano Lett. 17, 1455 (2017).
41 A. Chernikov, T. C. Berkelbach, H. M. Hill, A. Rigosi, Y. L. Li, O. B. Aslan, D. R. Reichman, M. S.
Hybertsen, and T. F. Heinz, Phys. Rev. Lett. 113, 076802 (2014).
42 M. D. McGehee and A. J. Heeger, Adv. Mater. 12, 1655 (2000).
43 A. Kohler, J. S. Wilson, and R. H. Friend, Adv. Mater. 14, 701 (2002).
44 A. Suna, Phys. Rev. B 1, 1716 (1970).
18
45 L. Luer, S. Hoseinkhani, D. Polli, J. Crochet, T. Hertel, and G. Lanzani, Nat. Phys. 5, 54 (2009).
46 Y. Z. Ma, L. Valkunas, S. L. Dexheimer, S. M. Bachilo, and G. R. Fleming, Phys. Rev. Lett. 94, 157402
(2005).
47 N. Kumar, Q. Cui, F. Ceballos, D. He, Y. Wang, and H. Zhao, Phys. Rev. B 89, 125427 (2014).
48 S. Mouri, Y. Miyauchi, M. Toh, W. Zhao, G. Eda, and K. Matsuda, Phys. Rev. B 90, 155449 (2014).
49 D. Sun, Y. Rao, G. A. Reider, G. Chen, Y. You, L. Brezin, A. R. Harutyunyan, and T. F. Heinz, Nano
Lett. 14, 5625 (2014).
50 S. Kar, Y. Su, R. R. Nair, and A. K. Sood, ACS Nano 9, 12004 (2015).
51 L. Yuan and L. B. Huang, Nanoscale 7, 7402 (2015).
52 Y.-Y. Lv, L. Xu, S.-T. Dong, Y.-C. Luo, Y.-Y. Zhang, Y. B. Chen, S.-H. Yao, J. Zhou, Y. Cui, S.-T. Zhang,
M.-H. Lu, and Y.-F. Chen, Phys. Rev. B 99, 195143 (2019).
53 Q. Cui, F. Ceballos, N. Kumar, and H. Zhao, ACS Nano 8, 2970 (2014).
19