Spectroscopic size and thickness metrics for liquid-exfoliated h-BN
Griffin, A., Harvey, A., Cunningham, B., Scullion, D., Tian, T., Shih, C-J., Gruening, M., Donegan, J. F., Backes,
C., Santos, E., & Coleman, J. N. (2018). Spectroscopic size and thickness metrics for liquid-exfoliated h-BN.
Chemistry of Materials
,
30
(6), 1998-2005. https://doi.org/10.1021/acs.chemmater.7b05188
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Chemistry of Materials
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Download date:09. Aug. 2022
Spectroscopic size and thickness metrics for liquid-exfoliated h-BN
Aideen Griffin,
1
Brian Cunningham,
2
Declan Scullion,
2
Tian Tian,
3
Chih-Jen Shih,
3
Myrta
Gruening,
2
Andrew Harvey,
1
John Donegan,
1
Elton J. G. Santos,
2
Claudia Backes,
4
Jonathan
N. Coleman
1*
1
School of Physics and CRANN & AMBER Research Centres, Trinity College Dublin, Dublin
2, Ireland
2
School of Mathematics and Physics, Queen's University Belfast, Belfast, BT71NN, United
Kingdom
3
Institute for Chemical and Bioengineering, ETH Zürich, 8093 Zürich, Switzerland
4
Chair of Applied Physical Chemistry, Ruprecht-Karls University Heidelberg, Im
Neuenheimer Feld 253, 69120 Heidelberg, Germany
*colemaj@tcd.ie
Abstract: For many 2D materials, optical and Raman spectra are richly structured, and
convey information on a range of parameters including nanosheet size and defect content. By
contrast, the equivalent spectra for h-BN are relatively simple, with both the absorption and
Raman spectra consisting of a single feature each, disclosing relatively little information.
Here, the ability to size-select liquid-exfoliated h-BN nanosheets has allowed us to
comprehensively study the dependence of h-BN optical spectra on nanosheet dimensions. We
find the optical extinction coefficient spectrum to vary systematically with nanosheet lateral
size due to the presence of light scattering. Conversely, once light scattering has been
decoupled to give the optical absorbance spectra, we find the size dependence to be mostly
removed save for a weak but well-defined variation in energy of peak absorbance with
nanosheet thickness. This finding is corroborated by our ab initio GW and Bethe Salpeter
equation calculations, which include electron correlations and quasiparticle self-consistency
(QSGW). In addition, while we find the position of the sole h-BN Raman line to be invariant
with nanosheet dimensions, the linewidth appears to vary weakly with nanosheet thickness.
These size-dependent spectroscopic properties can be used as metrics to estimate nanosheet
thickness from spectroscopic data.
Introduction
Hexagonal boron nitride (h-BN) is a layered material which is structurally analogous to
graphite.
1
Its physical properties resemble graphite in a number of ways, for example in its
high chemical stability, its large thermal conductivity and near superlative mechanical
properties. However, it is electrically very different to graphite, displaying a large bandgap
(5.5-6 eV) and negligible electrical conductivity.
Also like graphite,
2
h-BN can be produced in a 2-dimensional (2D) form by direct growth
3
as
well as by mechanical
4
and liquid phase exfoliation.
5-6
The exfoliated material retains the
properties of layered h-BN but in an ultra-thin, extremely flat morphology. This has resulted
in 2D h-BN being deployed in a range of applications. For example, due to its high bandgap
and extreme flatness, grown or mechanically exfoliated h-BN is widely used as a substrate or
encapsulating material for electronic devices based on other 2D materials such as graphene or
MoS
2
.
7-10
Alternatively, liquid-exfoliated h-BN nanosheets (which tend to be a few layers
thick and 100s of nm in length) have been used in a range of applications from reinforcing
6
or gas-barrier
11
fillers in polymer-based composites to thermally conductive inclusions
12
in
oils to dielectric materials in electronic devices
13-15
and electrochemical separators in
electrolytically gated transistors.
16
As with other 2D materials, the utility of h-BN in applications increases the importance of
our ability to characterize it. As with all 2D materials, basic characterization to measure
nanosheet size and thickness can be performed by transmission electron microscopy and
atomic force microscopy. However, statistical analysis of individual nanosheet measurements
using these techniques is time consuming and tedious. In contrast, optical spectroscopy
generally probes the ensemble and provides averaged information. However, compared to
other 2D nanomaterials optical spectroscopic characterization of h-BN has yielded much less
information. For example, while MoS
2
and WS
2
,
17-18
and to a lesser extent graphene,
19
have
information-rich optical absorption spectra which allow estimation of nanosheet size and
thickness, the absorption spectrum of h-BN appears to be information-poor, displaying few
features beyond a bandedge around 6 eV. Similarly, while the Raman spectra of MoS
2
and
graphene yield information about nanosheet dimensions
19-20
and defect content,
21
the h-BN
Raman spectrum contains a single line,
1
the properties of which have not been concisely
linked to any physical properties of the nanosheets. Although cathodoluminesce can give
information about nanosheet thickness,
22
these measurements are neither straightforward nor
widely accessible.
Here we shown that the absorption and Raman spectra of liquid-exfoliated BN-nanosheets are
not as bereft of information as has been previously thought. By performing optical
characterization of fractions of size-selected, liquid-exfoliated nanosheets, we show that the
extinction spectra are influenced by nanosheet lateral size while the nanosheet thickness can
be extracted from either the absorption or Raman spectra.
Results and Discussion
Size selection of BN
Liquid phase exfoliation is a versatile nanosheet production method which exfoliates layered
crystals down to few-layered nanosheets in appropriate stabilizing liquids.
23-24
It has been
applied to a range of layered crystals including graphite, h-BN
5-6, 25-29
and MoS
2
and tends to
yield polydisperse samples of nanosheets with broad lateral size (~100-1000 nm) and
thickness (~1-20 layers) distributions.
23, 30-31
As a result, centrifugation-based size selection is
required to enable any study where well-defined sizes are required. Such techniques range
from density gradient ultracentrifugation,
30
which gives fine size control at low yield, to
liquid cascade centrifugation (LCC),
18
which gives coarser size control at considerably higher
yield. Here we used LCC to size-select an as-prepared dispersion of BN nanosheets stabilized
in an aqueous sodium cholate solution (see Methods for exfoliation protocol and cascade
details), yielding fractions containing nanosheets of different lateral sizes and thicknesses.
In LCC, a dispersion is subjected to repeated centrifugation steps with successively
increasing centrifugal accelerations (expressed as relative centrifugal field, RCF, in units of
the earth’s gravitational field, g). After each centrifugation step, supernatant and sediment are
separated, the sediments are collected for analysis, while the supernatant is centrifuged at
higher centrifugal acceleration.
18
The sediments collected at low centrifugal acceleration
contain large/thick nanosheets, while the fractions collected at higher centrifugal acceleration
contain smaller and smaller nanosheets. This technique has a number of advantages; notably
that collecting the product as a sediment allows redispersion into a range of liquid
environments, simultaneously allowing solvent exchange and concentration increase. In
addition, very little material is wasted with up to 95% of exfoliated product distributed
among the fractions.
18
We label samples using the lower and upper centrifugation rates used in the preparation of
the fraction. For example, if the supernatant produced after centrifugation with
RCF=5,000×g-force (5k-g) is then centrifuged at 10,000 g and the sediment collected after
this step, we refer to the sample as 5-10 k-g.
Atomic Force Microscopy (AFM) was used to statistically analyze the nanosheet dimensions
for each fraction with representative images displayed in Figure 1 A. In each dispersion, 200-
350 nanosheets were measured, and their length (longest dimension), width (dimension
perpendicular to length) and thickness recorded. The nanosheet length data were plotted as
histograms with examples of the 0.4-1 k-g and 10-22 k-g fractions shown in Figure 1B. For
each fraction, the nanosheet length follows a lognormal statistical distribution with smaller
sizes obtained for increasing centrifugation speeds as expected. Additional histograms are
given in the SI (Figure S1). The mean nanosheet length is plotted versus the central g-value
(midpoint of high and low g-values used in the size selection) in figure 1C. Experimentally,
we found a roughly power-law decay of <L> with central g-value with an exponent close to -
0.5, similarly to other liquid-exfoliated 2D materials.
18
Some care must be taken when analyzing the statistical nanosheet-height data. This is
because the apparent AFM height of liquid-exfoliated nanosheets is typically larger than the
theoretical thickness of the nanosheets due to adsorbed/intercalated water and surfactant.
Similar to previous reports,
17, 32-33
we use step-height analysis to determine the apparent
thickness of a single monolayer by measuring the height of the terraces of partially exfoliated
nanosheets (Figure 1 D,E). Similar steps are then grouped and a plot of the mean step height
versus step height group number (Figure 1, F) leads to an apparent monolayer height of
0.99±0.01 nm, similar to the step height of 0.9 nm previously found for graphene.
32
Using
this information we can determine the number of layers, N, of the nanosheets allowing the
construction of histograms for each size-selected fraction. Typical histograms of the 0.4-1k g
and 10-22k g (Figure 1 F) samples show an increase in monolayer and few-layer nanosheets
and a narrowing of the distribution with increasing centrifugation speed. Histograms of all
other sizes are shown in the SI (Figure S1). The arithmetic mean values of nanosheet layer
number, <N>, is plotted versus the central g-force in figure 1H and shows significant
variation over the fractions from ~19 to 3.5. Experimentally, <N> followed a power-law with
an exponent of -0.4.
Atomic force microscopy (AFM) can be used to measure both nanosheet thickness and lateral
dimensions; this means, for each nanosheet of a given thickness, the volume can be estimated
