九州大学大学院総合理工学府報告第 44 巻 第 1 号 17-21 頁 令和 4 年 9 月

Engineering Sciences Reports, Kyushu University
Vol. 44, No. 1, pp. 17-21, SEP, 2022

Multilayer Boron Films and Band Gap Transition
Shahadat HOSSAIN*1, *4 Tensei IWAKI*2 and Takeshi NAKAGAWA*3,†
E-mail of corresponding author: nakagawa.takeshi.174@m.kyushu-u.ac.jp†(Received July 8, 2022, accepted July 19, 2022)
We report the growth of multilayer boron films on Mo(112) surface by e-beam deposition.
Subsequent annealing after deposition shows ordered structure of c(2×2) for 1st layer, which isconsistent with an earlier study. Additional deposition of boron atoms revealed no ordered structurefor 2nd and 3rd layer as evident from scanning tunneling microscopy and low energy electron
diffraction. Scanning tunneling spectroscopy measurements demonstrate the semiconducting natureof the 2nd and 3rd layer with a band gap of 1.2 and 1.4 eV, respectively. On the other hand, 1st layer
is metallic. These results show prospects for the practical applications of boron based nanomaterials
in the future.
Key words: Boron, Mo(112), Low energy electron diffraction(LEED), Scanning tunnelingmicroscopy(STM), Scanning tunneling spectroscopy(STS), Band gap.

1. Introduction
Intriguing properties such as low density,extreme hardness (comparable to diamond),
and melting point over 2000 °C have madeboron a potential material in a variety ofsectors, including high-temperature electronics
and lightweight reinforcing fillers.1,2) As aresult,extensive
studieshave
been
accomplished in recent years on the boronbased nanomaterials. For example, multilayerboron thin-film has shown promising prospects
as neutron detectors which may replacecommercially used 3He neutron detectors.3) In
another study, boron cluster has exhibited
higher capacity for storing Li and H2, which
offers to employ it as an anode material in Libatteries and H2 storage materials.4) The
photodetector device made of single-crystalline
ultrathin boron nanosheets shows highsensitivity, dependable stability, and quick
response, highlighting the material's enormous
potential for field emitters, interconnects,
*1
*2
*3
*4

Department of Molecular and Material Sciences,
Graduate Student
Department of Interdisciplinary Engineering Sciences,Graduate Student
Department of Advanced Materials Science andEngineering
Bangladesh Atomic Energy Commission

integratedcircuits,andoptoelectronicdevices.5)
Due to the lack of electrons in the valenceband, boron is a unique material withcomplicated structural characteristics. As aresult, all boron allotropes are made up oflarge-sized unit cells that include many atoms
in each case. The basic building block of these
allotropes is B12 icosahedra or icosahedral units.
It is assumed that the three valence electrons
are responsible for the bonding in B12icosahedra, where three neighboring boron
atoms form a triangle in which the electron
charge is concentrated.2,6) While boron may
create compounds with a wide range ofconfigurations, it only occurs in four
polymorphs as an element, which are αrhombohedral, β- rhombohedral, α-tetragonal,and β- tetragonal. The first two polymorphs arewell studied as they were found comparatively
stable whereas the latter ones are lessstudied.7) Plenty of studies have been found in
determining the electronic band nature of thesepolymorphs from various approaches either bytheoretical calculations or by experimentalobservations.8–13) A comparison between thesestudies revealed that the bulk boron allotropes
demonstrate a semiconducting nature with aband gap of 1.1 ~ 2.0 eV. However, variation inpressure and temperature induces metallicity

18

Multilayer Boron Films and Band Gap Transition

andsuperconductivity
insomeallotropes.12,14,15) While most of the findings are
based on bulk boron allotropes, electronic bandnature of ultra-thin layers and boron
nanostructures are comparatively rare. The
recentdevelopmentofboron
based
nanomaterials demands the search forelectronic nature and/or band gap of ultra-thinboron films for practical implementation.
To extend the knowledge of the energy bandof ultra-thin boron adsorbed layers on
refractory metal substrates, we haveinvestigated the band gap variation of boron
layers adsorbed on Mo(112) surface in detail.
Our previous work reported the formation of B 4
nanocluster on Mo(112) surface and metallic
nature of the cluster.16) Therefore, this study
concentrates on the additional boron layer'sformation on Mo(112) surface and revealing
electronic structures. Scanning tunnelingspectroscopy (STS) was used to measure thecorresponding band gap of boron layers, while
scanning tunneling microscopy (STM) and low
energy electron diffraction (LEED) were
employed to study the surface morphology andcrystallinity.

PtIr tips sometimes were covered with boron
during scanning, the tips were repeatedly
checked by measuring 𝐼 − 𝑉 spectra onMo(112) substrates, which shows characteristic
𝑑𝐼/𝑑𝑉 curves with parabolic shapes. Once thetip was covered by boron, the tip was cleaned
using 10 V pulse several times to peel off boronon it. For all the STS measurements, the set
point for bias voltage was at -1 V and tunnelingcurrent at 0.3 nA. The 𝑑𝐼/𝑑𝑉 curves arederived from numerical derivation of 𝐼 − 𝑉curves.
Mo(112) surface was cleaned by repeatedcycles of heating at 1200 °C in an oxygenpressure of 6×10-6 Pa to remove carbon-related
impurities and subsequently Ar+ ion sputteringand flash heating at ~1500 °C. We judged theMo(112) substrate to be clean when LEED
pattern showed (1×1) with low backgroundtypical for a clean surface.
Boron was deposited at room temperaturefrom a Ta crucible using an electron beam
evaporator. The samples were subsequently
annealed to improve the quality of boron films.
The annealing temperature required to obtainsmooth films depended on the film thickness

2. Experimental

3. Results and discussion

Experiments were performed in an ultrahigh vacuum chamber with a pressure of <2×10-8 Pa. STM and STS measurements wereaccomplished at room temperature withmechanically sharpened PtIr tips. Since theFig.1 shows the growth of boron films on
Mo(112) surface, which were prepared after
annealing at (a) 1200 °C for 0.8 ML, (b) 900 °Cfor 1.6 ML and (c) 900 °C for 3.2 ML. At 0.8 ML,
the surface shows clear and rather straight but

Fig.1 STM images for (a) 0.8, (b) 1.6 and (c) 3.2 ML of boron. The samples are prepared with boron deposited at room
temperature and subsequently annealed to 900 – 1200 °C. Corresponding LEED patterns are shown as insets of each
STM image (𝐸𝑝 = 70 eV) (d) Profile for 1.6 ML of boron along the white dashed line in (b).

令和 4 年度

九州大学大学院総合理工学報告第 44 巻 第 1 号

19Fig.2 STS spectra for (a), (b) one, (c), (d) two , and (e),(f) three layers boron films. (a), (c), (e):𝐼 − 𝑉 curves. (b),(d), (f):𝑑𝐼/𝑑𝑉 curves.

Band gap (eV)

1.5
1.0
0.5
0.0
0

1

2

3

4

Boron Layers
Fig.3 Band gap dependence on thickness of boron.

slightly corroded steps as shown in Fig.1(a). At
0.8 ML, 1 ML thick c(2×2) superstructuredeveloped, which coexists with clean Mo(112)
surface. LEED pattern in Fig. 1(a) confirms thepresence of c(2×2) superstructure. Theseresults are analogous to our previous report.16)
Fig.1(b) shows the STM image for 1.6 ML boron.
At 1.6 ML, boron completely covered thesurface, and regions with 1 ML and 2 ML
coexisted. From the line profile shown in
Fig.1(d), the surface shows a step height of 1.2
Å, almost identical to Mo(112) step height (1.28
Å), indicating that boron films grow layer-bylayer. With increasing boron coverage to 3.2 ML,the surface gets random (Fig.1(c)), but stillboron grows in a layer-by-layer fashion. Boronon Mo(112) grows layer-by-layer at least up toFig.4 (a,b) STM images of boron covered Mo(112)
surface after 3.2 ML deposition at room temperature.
Thus, the surface is covered by isolated boron nanoparticles (NP). Corresponding LEED pattern (𝐸𝑝 = 70eV) is shown as inset of STM image (a). (c) The profile
along the white dashed line in (b) shows very
corrugated surface.

3 ML. The LEED patterns for 1.6 and 3.2 MLshow no additional diffraction spots other thanthe substrate, indicating that boron films morethan 2 ML are disordered even after annealing
at 900 °C.
To get insights into electronic properties ofboron films, we have measured STS spectra on
1st, 2nd, and 3rd layers of boron. For all boron
thickness, 𝐼 − 𝑉 spectra are relatively flat
between 1 and −1 V and show steep uptake

20Multilayer Boron Films and Band Gap Transition

believe that the metallic one, NP-I in Fig.5(a)(b), comes from the area where boron does notcover, and the semiconductor one, NP-II inFig.5(c)-(d), from the area of boron clusters. Theband gap from boron NPs is ~ 1.3 eV, similar to
those for 2nd and 3rd layers of boron. Band gap
for nanoparticle depends on its size, thickness,
defects and so on, and is known to be larger
than that for bulk18). In our study, thedifference in the band gap is small between
NPs and films, and quantum effects in NPs are
not significant.

Fig.5 Two kinds of STS spectra measured on boron
nanoparticle (NP) covered Mo(112) surface after 3.2
ML deposition (Fig.4). (a), (b) NP-I shows metallic
nature whereas (c), (d) NP-II shows non-metallic.

beyond |𝑉| > 1 eV (Fig.2(a), (c), (e)). It isdifficult to evaluate the band gap from the 𝐼 −𝑉 curves. On the other hand, 𝑑𝐼/𝑑𝑉 spectra
mark band gap for 2nd and 3rd layer boron asshown in Fig. 2(d) and (f), respectively. Notethat on a single boron layer, 𝑑𝐼/𝑑𝑉 spectrum
show characteristic feature of a metallic sample
as shown in Fig. 2 (b), which is similar to thatfor Mo(112) substrate.16)
According to band structure calculations andoptical absorption spectroscopy, boron films
were known to be a semiconductor with a band
gap of 1.1 ~ 2.0 eV.8–13,17) Fig. 3 shows
evaluation of the band gap for multilayer boron,
1.2 eV for 2nd and 1.4 eV for 3rd layer, which
agrees with previous reports,8–13,17) and theband gap at 1st layer is zero. The band gap
formation at the 2nd layer boron indicates thatthe 1st layer boron is effectively connected toMo(112) substrate and decouples the 2nd layerfrom the substrate.
We have also tried to measure the STS for theas-deposited boron on Mo(112) surface at room
temperature. Boron forms nanoparticles (NPs)on Mo(112) when deposited at room
temperature, as shown in Fig.4(a). An Enlarged
STM image is shown in Fig.4(b), where the sizeof boron particles is 1-3 nm in width and ~ 0.4
nm in height. NPs do not coalesce at room
temperature. We have measured the STS on
the as-deposited sample and found two kinds ofspectra. One is metallic, and the other issemiconductor-like, as shown in Fig.5.
Although STS measurements have low spatial
resolution on this sample because of themeasurement at room temperature and theclosely arranged boron nanoparticles, we

4. Summary and Conclusion
Boron films prepared on Mo(112) surface
show layer-by-layer growth at least up to 3 ML,
and a band gap above 2 ML. 1st layer boron
shows ordered structure, c(2×2), and isrevealed as metallic. Boron nanoparticles also
impart to have band gap similar to those for2nd and 3rd layers. The band gap information
is particularly interesting for future boron
based nanomaterials fabrication.
Acknowledgments
This research was partially supported by
Grants-in-Aidfor
Scientific
Research(KAKENHI)(No. 22K04928). S. H. is supportedby the "Innovative Asia Initiative" program by
Japan International Cooperation Agency(JICA) and funded by the Ministry of Education,
Culture, Sports, Science, and Technology(MEXT) scholarship.
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