Although germanium (Ge) is a semiconductor frequently used in many facets of materials science, its optical applications are limited because of an indirect band structure, which significantly diminishes absorption and emission efficiency. However, sufficiently high levels of tin (Sn) alloying enable an indirect-to-direct band structure crossover, resulting in improved optical properties. Moreover, the bandgap of GeSn alloys can be tuned by simply varying the alloy composition; therefore, the material can be modified for compatibility with silicon (Si) based electronics. While lattice mismatch makes the solubility of Sn in Ge extremely low in bulk alloys (<1%), metastable nanoalloys produced under nonequilibrium conditions show minimum to no lattice strain, allowing the synthesis of GeSn nanoalloys with wider tunability of Sn (up to 95%). Furthermore, the size-tunable confinement energy characteristic of GeSn nanoalloys has been shown to greatly increase the energy gaps, resulting in tunable visible to near-IR optical properties. Herein, the authors summarize recent advances in the synthesis of 0D and 1D GeSn alloy nanostructures and their emerging physical properties in light of their potential applications in advanced electronic and photonic technologies.

Elemental germanium is a fairly abundant, low toxicity semiconductor that is crucially important to materials science and engineering disciplines. It has been a fundamental building block of many electronic devices since it was used to make the first working transistor at Bell Labs in 1948. Currently, it is a common constituent of solar cells, photodetectors, and waveguides.1 Germanium also has a larger Bohr radius, larger intrinsic carrier concentration, and smaller effective electron mass compared to its group IV neighbor, silicon.2 Additionally, germanium is significantly less toxic than other popular semiconductors such as CdSe, InAs, and PbSe. This is important because, for many applications of semiconductors, specifically bioimaging, there has been a concerted effort to substitute heavy metal species for lighter, more benign materials.3–5 However, there are a few obstacles that have prevented the applicability of germanium in photonic technologies, most famously the indirect fundamental bandgap.6 This unfavorable band structure leaves germanium with an absorption coefficient significantly lower than that of any direct gap material. There have been many recent efforts to engineer the electronic structure of germanium so as to create a direct fundamental bandgap. This is typically done by applying tensile strain to the crystal, which has been shown to reduce the energy of the conduction band.7,8 When tensile strain is applied, the direct gap (Г) decreases faster than the indirect gap (L), resulting in an indirect-to-direct crossover.9,10 Although many reports seek to create strain through epitaxial growth on a substrate with a large lattice constant, crossover can also be achieved through alloying with a large group IV atom such as tin.11–14 Alloying with α-Sn not only promotes the indirect-to-direct crossover but also allows wider tunability of bandgap as a function of tin composition.15,16 However, the solubility of tin in bulk germanium is extremely low (<1%) because of alloy-induced lattice strain.17 This obstacle has inspired the incorporation of tin into germanium-based 0D and 1D nanostrucutres produced via nonequilibrium synthetic methods, which reveal a much higher strain tolerance, exceptionally high Sn (up to 95%) solubility and consequently unique physical properties.

Semiconductor nanocrystals (NCs), which show strong absorption and emission of visible to near-IR (NIR) photons, have gained interest because of their inexpensive solution-based synthesis and processing, exceptional photostability, and unique size confinement effects that allow facile tuning of the electronic structure.18,19 Additionally, NCs can potentially exhibit fascinating and incredibly useful properties such as “carrier multiplication,” also called “multiple exciton generation” (MEG), where the material will absorb a single ultraviolet photon and use the energy to create multiple electron-hole pairs,20,21 significantly increasing the efficiency of optical devices such as solar photovoltaics. Although narrow-gap Ge NCs have the advantages of solution processability and low-temperature synthesis, GeSn alloy NCs demonstrate added benefits of direct gap behavior and wider tunability of energy gaps.

In this review, we will evaluate recent advances in the synthesis, modeling, and applications of GeSn nanomaterials. Although other GeSn reviews have focused on epitaxially grown thin films,22,23 these methods lack the advantages of inexpensive solution-based processing and low-temperature synthesis that are characteristic of conventional, tabletop chemistry. Additionally, thin films are bulk materials and, thus, cannot be used to utilize the effects of quantum confinement, such as additional bandgap tunability and MEG. And so, this report will concentrate on 0D and 1D materials synthesized through colloidal chemistry methods and their notable physiochemical properties.

The earliest paper reporting the synthesis of homogeneous GeSn alloy NCs small enough to display appreciable quantum confinement effects was reported by Esteves et al.24 Here, NCs from 3.4–4.6 and 15–23 nm were colloidally synthesized via reduction of precursor halides, GeI2 and SnCl2, by n-butyllithium in oleylamine (OLA) at 300 °C (Figs. 1 and 2), where Sn compositions of 0%–27.9% were achieved. Thermal stability tests showed resistance to phase segregation up to 500 °C. Additionally, the smaller, quantum-confined alloys had bandgaps well within the near-IR range (0.75–1.29 eV), significantly higher than bulk values of 0.35–0.80 eV.12 

FIG. 1.

Scheme for the synthesis of GeSn NCs. Here, Ge and Sn halide precursors are reduced by OLA and n-butyllithium, creating seed nuclei that grow into homogeneous alloys. Reprinted with permission from Esteves et al., Chem. Mat. 27, 1559 (2015). Copyright 2015, American Chemical Society.

FIG. 1.

Scheme for the synthesis of GeSn NCs. Here, Ge and Sn halide precursors are reduced by OLA and n-butyllithium, creating seed nuclei that grow into homogeneous alloys. Reprinted with permission from Esteves et al., Chem. Mat. 27, 1559 (2015). Copyright 2015, American Chemical Society.

Close modal
FIG. 2.

TEM images of Ge1xSnx alloy NCs with varied Sn compositions. (a) x = 0.018, (b) x = 0.046, (c) x = 0.066, and (d) x = 0.236. (e) Dark field TEM of Ge0.87Sn0.13 NC along with energy dispersive spectroscopy elemental maps of (f) Ge, (g) Sn, and (h) Ge and Sn overlay. Reproduced with permission from Esteves et al., Chem. Commun. 52, 11665 (2016). Copyright 2016, The Royal Society of Chemistry.

FIG. 2.

TEM images of Ge1xSnx alloy NCs with varied Sn compositions. (a) x = 0.018, (b) x = 0.046, (c) x = 0.066, and (d) x = 0.236. (e) Dark field TEM of Ge0.87Sn0.13 NC along with energy dispersive spectroscopy elemental maps of (f) Ge, (g) Sn, and (h) Ge and Sn overlay. Reproduced with permission from Esteves et al., Chem. Commun. 52, 11665 (2016). Copyright 2016, The Royal Society of Chemistry.

Close modal

GeSn NC synthesis can be challenging and must be performed exactly as described to avoid the formation of undesirable byproducts, the most common of which being β-Sn and GeO2. Fortunately, the formation of stable β-Sn nuclei can be avoided with a fast, butyllithium-catalyzed precursor reduction at exactly 230 °C, just below the melting point of Sn. Oxidation and the appearance of GeO2 can be avoided by carefully degassing and backfilling the flask with N2 gas before heat is applied. It is important that N2, supplied via the Schlenk line, be constantly flowing through the reaction after the heating mantle has been turned on. Size and shape dispersity can be minimized by removing heat from the reaction exactly when indicated, as heating the reaction for too long can result in size distribution broadening via Ostwald ripening.25,26 Finally, x-ray diffraction (XRD) measurements can be used to determine whether an appreciable amount of β-Sn and/or GeO2 has been formed.

Ramasamy et al.27 reported the colloidal synthesis of 5–15 nm GeSn NCs (Fig. 3) with Sn compositions of 0%–42%. This process is similar to that published by Esteves et al.,24 except that this synthesis used a different Sn precursor [Sn(HMDS)2] and occurred at a slightly lower temperature (250 °C). While Sn compositions greater than 18.0% required n-butyllithium as a reducing agent, lower compositions could be achieved using only the solvent, OLA, for reduction. Additionally, a microwave-based synthetic method was introduced, though the microwaved product had more size and shape dispersity than the product obtained through conventional heating. Finally, shifts in 119Sn nuclear magnetic resonance (NMR) suggested the formation of a heterometallic Ge–Sn–N complex preceding NC nucleation.

FIG. 3.

TEM images of 10 nm Ge1xSnx NCs. (a) x = 0.32 and (b) x = 0.36. (c) and (d) Images of (a) and (b) with higher magnification. Inset shows high resolution TEM image (HRTEM). (e) and (f) Inverse fast Fourier transform of HRTEM images in (c) and (d). The line inset reveals lattice spacings along the [111] direction. Reprinted with permission from Ramasamy et al., Chem. Mat. 27, 4640 (2015). Copyright 2015, American Chemical Society.

FIG. 3.

TEM images of 10 nm Ge1xSnx NCs. (a) x = 0.32 and (b) x = 0.36. (c) and (d) Images of (a) and (b) with higher magnification. Inset shows high resolution TEM image (HRTEM). (e) and (f) Inverse fast Fourier transform of HRTEM images in (c) and (d). The line inset reveals lattice spacings along the [111] direction. Reprinted with permission from Ramasamy et al., Chem. Mat. 27, 4640 (2015). Copyright 2015, American Chemical Society.

Close modal

Further mechanistic details on GeSn NC synthesis were reported in another paper by Ramasamy et al.,28 where unprecedentedly high (up to 95%) Sn compositions were reported. Here, further NMR studies suggested that the process begins through the formation of Sn and Ge cubane structures with each metal atom surrounded by four nitrogen atoms. From here, heterometallic complexes form, then nucleating into β-Sn clusters with Ge atoms adsorbed on the surface. Time and temperature allow the evolution from this precursor into homogenous alloy particles. Thermal stability up to 500 °C was achieved, though this required surrounding the NCs with a layer of sulfur to slow Sn diffusion.

Yang et al.29 developed a low-temperature (60–180 °C) synthesis of 11 nm GeSn NCs with 18.4%–62.1% Sn composition. This was done by using previously synthesized Sn NCs in OLA and injecting GeI2 precursors, which are reduced to Ge0 by the solvent and diffuse into Sn NCs with heat and time. This synthesis (Fig. 4) is especially novel, as it differs from all other colloidal GeSn NC syntheses with an entirely diffusion-based mechanism and an independently synthesized pure β-Sn precursor. Transmission electron microscope (TEM) showed the shrinking crystalline β-Sn domains as Ge diffused inward, creating an amorphous GeSn region that increased in crystallinity over time. Bandgaps fall in the range of 0.51–0.72 eV for the as-prepared alloys and vary with the Sn content (18%–27%) in agreement with previous reports. Additionally, the particles synthesized through this low-temperature method display a more uniform spherical shape compared to those synthesized via higher temperature methods.

FIG. 4.

Mechanism proposed by Yang and co-workers. Reprinted with permission from Yang et al., Chem. Mat. 31, 2248 (2019). Copyright 2019, American Chemical Society.

FIG. 4.

Mechanism proposed by Yang and co-workers. Reprinted with permission from Yang et al., Chem. Mat. 31, 2248 (2019). Copyright 2019, American Chemical Society.

Close modal

GeSn nanowires (NWs) of 190 ± 30 nm in diameter and 12.4% Sn composition were synthesized by Barth et al. (Fig. 5).30 These were produced using Sn(HMDS)2 and Ge(HMDS)2 precursors in dodecylamine at 230 °C via microwave synthesis. The product precipitated as a solid and was easily collected via centrifugation. These NWs had high variation in length, roughly from 1 to 4 μm. Mechanistically (Fig. 6), the author describes heterometallic imido cubane complexes preceding crystal nucleation, similar to those described in mechanistic studies of GeSn NC formation.28 Energy dispersive x-ray analysis showed that the solubility of Sn in Ge is inversely proportional to the NW diameter, ranging from 10.7% Sn in 270 nm NWs to 28.4% in 10 nm NWs.31 

FIG. 5.

Illustration of the cubane intermediates preceding GeSn seed nuclei formation and NW growth. Reprinted with permission from Seifner et al., Chem. Mat. 27, 6125 (2015). Copyright 2015, American Chemical Society.

FIG. 5.

Illustration of the cubane intermediates preceding GeSn seed nuclei formation and NW growth. Reprinted with permission from Seifner et al., Chem. Mat. 27, 6125 (2015). Copyright 2015, American Chemical Society.

Close modal
FIG. 6.

Scanning electron microscopy images of colloidal synthesized GeSn NWs. Reproduced with permission from Barth et al., Chem. Commun. 51, 12282 (2015). Copyright 2015, The Royal Society of Chemistry.

FIG. 6.

Scanning electron microscopy images of colloidal synthesized GeSn NWs. Reproduced with permission from Barth et al., Chem. Commun. 51, 12282 (2015). Copyright 2015, The Royal Society of Chemistry.

Close modal

Seifner et al. reported the synthesis of GeSn nanorods (NRs), which are morphologically similar to NWs, with 0.5–1.0 μm in length.32 This synthetic procedure is identical to the one published by Barth et al.,30 except that microwave time is kept under 2 min to produce short rods. The NR XRD patterns obtained after the first 60 s of the reaction were dominated by β-Sn peaks, suggesting that NR growth starts with the nucleation of pure β-Sn particles. Sharp diffraction peaks corresponding to Ge appeared in the product NRs after 2 min growth time at 140 °C and shifted to lower 2θ angles due to lattice expansion from Sn incorporation. TEM pictures after 2 min show a rod structure appear after elongation in one direction with a bulb of β-Sn remaining at the nucleation site.

Photoluminescence (PL) maxima of GeSn NCs range from low visible to IR wavelengths and vary as a function of both size and Sn composition. Esteves et al. reported GeSn NCs with orange-red PL maxima from 1.72 to 2.05 eV,26 which were corroborated by modeling via density functional theory (DFT). This was done with extremely small NCs (1–3 nm in diameter) and Sn compositions of 1.8%–23.6%. A separate range of tunability, from 1.31 to 1.61 eV, was reported in a later study,33 allowing for compatibility with near-IR technologies (Fig. 7).

FIG. 7.

PL measurements performed on ultrasmall (1.8–2.2 nm) Ge1xSnx NCs of varied composition: (1) x = 1.5% (1.62 eV), (2) x = 1.9% (1.52 eV), (3) x = 2.7% (1.43 eV), (4) x = 3.4% (1.38 eV), (5) x = 4.2% (1.34 eV), and (6) x = 5.6% (1.31 eV). These maxima are ideal for applications in NIR or low visible photonics. Reproduced with permission from Tallapally et al., Nanoscale 10, 20296 (2018). Copyright 2018, The Royal Society of Chemistry.

FIG. 7.

PL measurements performed on ultrasmall (1.8–2.2 nm) Ge1xSnx NCs of varied composition: (1) x = 1.5% (1.62 eV), (2) x = 1.9% (1.52 eV), (3) x = 2.7% (1.43 eV), (4) x = 3.4% (1.38 eV), (5) x = 4.2% (1.34 eV), and (6) x = 5.6% (1.31 eV). These maxima are ideal for applications in NIR or low visible photonics. Reproduced with permission from Tallapally et al., Nanoscale 10, 20296 (2018). Copyright 2018, The Royal Society of Chemistry.

Close modal

Details of the GeSn NC energy gap and carrier dynamics were investigated by Hafiz et al.34 Here, steady state and time resolved PL spectroscopy were used to investigate the carrier dynamics of 2.0 ± 0.8 nm NCs at 295 and 15 K. Because of extreme quantum confinement effects, NCs of 1.8–2.2 nm size exhibit tunable bandgaps (1.61–1.88 eV) significantly larger than GeSn thin films (0.35–0.80 eV) with the same Sn composition. At 15 K, the PL intensity peaks displayed a minor blueshift relative to room temperature, attributed to exchange splitting between dark and bright exciton states. Transient PL measurements were fit with biexponential decay functions, which distinguished the fast, radiative surface recombination (time constant = τfast = 4 μs) from the slower core recombination (τslow = 24 μs). Total decay time was measured to be approximately 3–27 μs at 15 K, with radiative pathways dominated by spin-forbidden dark excitons and surface states. There was a significant decrease in decay time to 9–28 ns at 295 K (τfast = 1–2 μs, τslow = 10–20 μs), because of the thermal activation of spin-allowed bright exciton states and carrier detrapping from surface states (Fig. 8).

FIG. 8.

Radiative recombination pathways for 2.0 ± 0.8 nm Ge0.94Sn0.06 NC alloys at 295 K. The time constants for surface (1–2 μs) and core (10–20 μs) recombination are lower than those from the same study taken at 15 K (4 and 24 μs, respectively). Reprinted with permission from Hafiz et al., J. Phys. Chem. Lett. 7, 3295 (2016). Copyright 2016, American Chemical Society.

FIG. 8.

Radiative recombination pathways for 2.0 ± 0.8 nm Ge0.94Sn0.06 NC alloys at 295 K. The time constants for surface (1–2 μs) and core (10–20 μs) recombination are lower than those from the same study taken at 15 K (4 and 24 μs, respectively). Reprinted with permission from Hafiz et al., J. Phys. Chem. Lett. 7, 3295 (2016). Copyright 2016, American Chemical Society.

Close modal

Further analysis and quantification of dark-bright exciton splitting were accomplished by Demchenko et al.35 Here, it was confirmed that the dark-bright splitting energy decreases with an increasing diameter as well as increasing Sn incorporation. The smallest NCs studied (∼1.4 nm) were determined to have a splitting of 80–40 meV for Sn compositions of 0%–20%, while this range is reduced to 40–20 meV for the larger 2–3 nm NCs. Hybrid functional calculations (HSEs) were also used to measure the oscillator strengths of optical transition across the energy gap as a function of Sn incorporation. In pure 1.4 nm Ge NCs, the oscillator strength of the bright state was found to be five orders of magnitude higher than its dark counterpart, but this same difference was reduced to nearly two orders of magnitude in 1.4 nm NCs with ∼20% Sn incorporation. Additionally, HSE calculations were found to slightly deviate from the experiment in predicting energy gaps as a function of NC diameter. This was attributed to imperfect surface passivation, which has been known to affect the optical properties of many nanocrystalline systems. HSE theory has been shown to reproduce the directly measured properties (energy gaps, lattice constants, and absorption/PL spectra) of 1.8–2.2 nm GeSn NCs with high accuracy. Unlike bulk lattice constants, which show significant bowing,36 averaged lattice constants calculated for ∼2.1 nm GeSn NCs show essentially linear Vegard's behavior, in agreement with experiments, suggesting production of strain-free quantum-confined alloys.

In 2019, Baira et al. published a series of theoretical papers37–39 on the effects of morphology and external electric field on optical transitions and electronic properties of 15–40 nm GeSn NCs. Here, all results were obtained through numerically solving the Schrödinger equation by the finite element method. When studying nanostructures of a dome or pyramidal shape,38 it was found that an indirect-to-direct transition could be achieved by varying the base diameter and the aspect ratio of the particle. For both of these shapes, the minimum diameter to allow indirect-to-direct crossover was inversely proportional to the aspect ratio, and this minimum was lower in pyramidal particles compared to the dome shaped species. The pyramids also display a higher oscillator strength for the optical bandgap as well as a longer radiative recombination time. A separate study explained how the electron density in pyramidal nanostructures can be slightly displaced by an external electric field. This has important consequences for the optical properties of the species, as electronic transition energy decreases if an electric field directs electron density toward the base of the pyramid (positive bias), but the opposite effect occurs if a field concentrates electrons at the tip of the structure (negative bias). Applying a positive bias will also increase the intersubband dipole moment, while a negative bias will decrease it. Taken together, these findings suggest there may be much to gain by investigating GeSn NC structure-property relationships.

A study by Kosmaca et al.48 investigated the mechanical properties of GeSn NWs with 115 nm radius and 15 μm length, identifying them as potential candidates for electromechanical switches. These NWs were found to have a mechanical resonant frequency of 674 kHz when one side was clamped to a support, as well as a quality factor of Q ≈ 1000. The Q value is reported to be directly proportional to NW thickness, in agreement with similar studies on pure Ge NWs.40 These results reaffirm the correlation between the surface area and energy dissipation/loss of vibrational frequency. Additionally, the NWs' Young modulus was found to increase with decreasing radii, reaching an asymptotic height at 30 nm, though the authors admit to possible error, as factors contributing to mechanical properties at the nanoscale are still poorly understood.

GeSn nanostructures have numerous potential applications in batteries, bioimaging, photovoltaics, and photocatalysis. While many solar cells and photovoltaic technologies are made with organic dye molecules, organics are fundamentally unstable due to photobleaching, which is not a concern with the stable, sturdy, inorganic nature of NC-based solar cells.41 This is in addition to the aforementioned direct bandgap and MEG behavior present in germanium-based NCs. The resistance of NCs to photobleaching may also help them replace organic fluorophores in bioimaging. This has already been done with ZnS coated CdSe NCs, which can be functionalized with an antibody to target a specific protein or complex, even in living neural cells.42 Finally, many semiconducting NCs have been used as photocatalysts. Generally, this is done by applying a constant light source to the NC, creating a self-sustaining population of excited electrons, which can be used as reductants, and electron holes, which can be used as oxidants. This method has been applied to important reactions such as C–C bond formation in organic molecules43 and hydrogen gas production44 although the NCs used in these papers contained highly toxic heavy metal species, a problem that would be solved upon application of GeSn technology.

One-dimensional ∼10 nm graphite coated GeSn NCs were evaluated as potential Li battery anode materials in a study by Cho et al.45 The most effective alloys for battery applications were NCs with 5.0% Sn incorporation. XRD showed that these particles were free of β-Sn impurities. The electrochemical analysis gave charge/discharge capacities of 1470 and 990 mAh/g. This resulted in an initial coulombic efficiency of 67%. The authors described the formation of a solid electrolyte interphase on the electrode surface, possibly decreasing the initial coulombic efficiency.

Synthesizing core-shell structures is a well-known method to passivate the surface, diversify functionality, and increase the PL intensity of core NCs.46 Boote et al.49 synthesized 11–13 nm Ge0.95Sn0.05@CdS core-shell structures to improve the luminescent properties of the CdS layer. The product displayed a PL peak at ∼950 nm with a 15-fold increase in intensity relative to a similar Ge@CdS structure. This improvement was attributed to the expanded lattice constant of the GeSn alloy species, which is closer to that of CdS compared to the smaller Ge lattice, thus resulting in more effective epitaxy between the core and shell.

Another important application of group IV NCs is near-IR photodetection. Slav et al. reported the synthesis of a photodetecting material made from 10 to 20 nm GeSn NCs embedded in an SiO2 matrix.47 This was done through magnetron cosputtering of Ge, Sn, and SiO2 followed by a dynamic annealing step. The Sn composition of the NCs was tunable in the range of 9.0%–22.0% by varying growth and annealing temperatures. Photocurrent measurements showed that the GeSn layer could detect 1200–2200 nm radiation, although the wavelength range varies significantly with the Sn composition.

This review summarizes recent breakthroughs in GeSn nanotechnology. Synthetic advancements have allowed the colloidal synthesis of NCs from 15 nm to as small as 1.4 nm, with Sn composition of 0.0%–95.0%, allowing for a wide range of bandgap tunability through the low visible and NIR range. DFT calculations and temperature dependent PL studies have given details on the electronic structure and verified exchange splitting between dark and bright exciton states. Additionally, synthetic methods have been developed to produce direct gap GeSn NWs, which have additional functionality, for example, in a Li-ion battery anode material due to their high surface area and carrier mobility. Furthermore, the door has been opened to functionalizing these materials through core-shell synthesis or by embedding in a dielectric matrix. The past several years of research have demonstrated the extensive utility of GeSn nanotechnology.

The authors gratefully acknowledge the Department of Chemistry, Virginia Commonwealth University and U.S. National Science Foundation (NSF) (No. DMR-1506595) for financial support.

1.
C.
Claeys
and
E.
Simoen
,
Germanium-Based Technologies From Materials to Devices
(
Elsevier Science
,
Amsterdam
,
2007
).
2.
S. M.
Sze
,
Physics of Semiconductor Devices
, 3rd ed. (
Wiley
,
Hoboken
,
NJ
,
2007
).
3.
M. A.
Walling
,
J. A.
Novak
, and
J. R. E.
Shepard
,
Int. J. Mol. Sci.
10
,
441
(
2009
).
4.
J.
Fan
and
P. K.
Chu
,
Small
6
,
2080
(
2010
).
5.
R. R.
Dietert
,
J. E.
Lee
,
I.
Hussain
, and
M.
Piepenbrink
,
Toxicol. Appl. Pharm.
198
,
86
(
2004
).
6.
T. M.
Razykov
,
C. S.
Ferekides
,
D.
Morel
,
E.
Stefanakos
,
H. S.
Ullal
, and
H. M.
Upadhyaya
,
Sol. Energy
85
,
1580
(
2011
).
7.
Y.
Lee
,
S. B.
Cho
, and
Y. C.
Chung
,
ACS Appl. Mater. Interfaces
6
,
14724
(
2014
).
8.
H. J.
Conley
,
B.
Wang
,
J. I.
Ziegler
,
R. F.
Haglund
, Jr.
,
S. T.
Pantelides
, and
K. I.
Bolotin
,
Nano Lett.
13
,
3626
(
2013
).
9.
S.
Gupta
,
B.
Magyari-Koepe
,
Y.
Nishi
, and
K. C.
Saraswat
,
J. Appl. Phys.
113
,
073707
(
2013
).
10.
S.
Gupta
,
R.
Chen
,
Y. C.
Huang
,
Y.
Kim
,
E.
Sanchez
,
J. S.
Harris
, and
K. C.
Saraswat
,
Nano Lett.
13
,
3783
(
2013
).
11.
S.
Wirths
 et al,
Appl. Phys. Lett.
102
,
4
(
2013
).
12.
R.
Ragan
,
K. S.
Min
, and
H. A.
Atwater
,
Mat. Sci. Eng. B
87
,
204
(
2001
).
13.
M. H.
Lee
,
P. L.
Liu
,
Y. A.
Hong
,
Y. T.
Chou
,
J. Y.
Hong
, and
Y. J.
Siao
,
J. Appl. Phys.
113
,
194507
(
2013
).
14.
P.
Moontragoon
,
Z.
Ikonic
, and
P.
Harrison
,
Semicond. Sci. Technol.
22
,
742
(
2007
).
15.
V. R.
D’Costa
,
C. S.
Cook
,
A. G.
Birdwell
,
C. L.
Littler
,
M.
Canonico
,
S.
Zollner
,
J.
Kouvetakis
, and
J.
Menendez
,
Phys. Rev. B
73
,
12
(
2006
).
16.
K. S.
Min
and
H. A.
Atwater
,
Appl. Phys. Lett.
72
,
1884
(
1998
).
17.
C.
Kittel
,
Introduction to Solid State Physics
, 8th ed. (
Wiley
,
Hoboken
,
NJ
,
2005
).
18.
C. B.
Murray
,
C. R.
Kagan
, and
M. G.
Bawendi
,
Annu. Rev. Mater. Sci.
30
,
545
(
2000
).
19.
T.
Trindade
,
P.
O’Brien
, and
N.
Pickett
,
Chem. Mat.
13
,
3843
(
2001
).
20.
S.
Saeed
,
C.
de Weerd
,
P.
Stallinga
,
F.
Spoor
,
A.
Houtepen
,
L.
Siebbeles
, and
T.
Gregorkiewicz
,
Light Sci. Appl.
4
,
e251
(
2015
).
21.
K.
Hyeon-Deuk
and
O. V.
Prezhdo
,
J. Phys. Condens. Matter
24
,
36
(
2012
).
22.
X.
Liu
,
G. B.
Braun
,
M.
Qin
,
E.
Ruoslahti
, and
K. N.
Sugahara
,
Nat. Commun.
8
,
343
(
2017
).
23.
S.
Zaima
,
O.
Nakatsuka
,
N.
Taoka
,
M.
Kurosawa
,
W.
Takeuchi
, and
M.
Sakashita
,
Sci. Technol. Adv. Mater.
16
,
4
(
2015
).
24.
R. J.
Alan Esteves
,
M.
Ho
, and
I. U.
Arachchige
,
Chem. Mater.
27
,
1559
(
2015
).
25.
N. G.
Bastús
,
J.
Comenge
, and
V.
Puntes
,
Langmuir
27
,
11098
(
2011
).
26.
R. J.
Alan Esteves
,
S.
Hafiz
,
D. O.
Demchenko
,
U.
Özgür
, and
I. U.
Arachchige
,
Chem. Commun.
52
,
11665
(
2016
).
27.
K.
Ramasamy
,
P. G.
Kotula
,
A.
Fidler
,
M.
Brumbach
,
J.
Pietryga
, and
S. A.
Ivanov
,
Chem. Mater.
27
,
4640
(
2015
).
28.
K.
Ramasamy
,
P. G.
Kotula
,
N.
Modine
,
M. T.
Brumbach
,
J. M.
Pietryga
, and
S. A.
Ivanov
,
Chem. Commun.
55
,
2773
(
2019
).
29.
Q.
Yang
 et al,
Chem. Mater.
31
,
2248
(
2019
).
30.
S.
Barth
,
M. S.
Seifner
, and
J.
Bernardi
,
Chem. Commun.
51
,
12282
(
2015
).
31.
M. S.
Seifner
,
F.
Biegger
,
A.
Lugstein
,
J.
Bernardi
, and
S.
Barth
,
Chem. Mater.
27
,
6125
(
2015
).
32.
M. S.
Seifner
,
S.
Hernandez
,
J.
Bernardi
,
A.
Romano-Rodriguez
, and
S.
Barth
,
Chem. Mater.
29
,
9802
(
2017
).
33.
V.
Tallapally
,
T. A.
Nakagawara
,
D. O.
Demchenko
,
U.
Özgür
, and
I. U.
Arachchige
,
Nanoscale
10
,
20296
(
2018
).
34.
S. A.
Hafiz
,
R. J. A.
Esteves
,
D. O.
Demchenko
, and
I. U.
Arachchige
,
J. Phys. Chem. Lett.
7
,
3295
(
2016
).
35.
D.
Demchenko
,
V.
Tallapally
,
R.
Esteves
,
S.
Hafiz
,
T. A.
Nakagawara
,
I.
Arachchige
, and
U.
Ozgur
,
J. Phys. Chem. C
121
,
18299
(
2017
).
36.
C.
Eckhardt
,
K.
Hummer
, and
G.
Kresse
,
Phys. Rev. B
89
,
165201
(
2014
).
37.
B.
Mourad
,
S.
Bassem
,
M.
Niyaz Ahamad
, and
I.
Bouraoui
,
Micromachines
10
,
243
(
2019
).
38.
B.
Mourad
,
A.
Maha
,
S.
Bassem
,
M.
Niyaz Ahmad
, and
I.
Bouraoui
,
Results Phys.
12
,
1732
(
2019
).
39.
M.
Baira
,
B.
Salem
, and
B.
Ilahi
,
Nanomaterials
9
,
124
(
2019
).
40.
D.
Smith
,
V.
Holmberg
,
D. C.
Lee
, and
B. A.
Korgel
,
J. Phys. Chem. C
112
,
10725
(
2008
).
41.
S.
Sfaelou
,
A. G.
Kontos
,
L.
Givalou
,
P.
Falaras
, and
P.
Lianos
,
Catal. Today
230
,
221
(
2014
).
42.
Y.
Wang
 et al,
Bioconjugate Chem.
25
,
2205
(
2014
).
43.
J. A.
Caputo
,
L. C.
Frenette
,
N.
Zhao
,
K. L.
Sowers
,
T. D.
Krauss
, and
D. J.
Weix
,
J. Am. Chem. Soc.
139
,
4250
(
2017
).
44.
P.
Wang
,
M.
Wang
,
J.
Zhang
,
C.
Li
,
X.
Xu
, and
Y.
Jin
,
ACS Appl. Mater. Interfaces
9
,
35712
(
2017
).
45.
Y. J.
Cho
 et al,
Phys. Chem. Chem. Phys.
15
,
11691
(
2013
).
46.
N.
Kumar
and
S.
Kumbhat
,
Essentials in Nanoscience and Nanotechnology
(
Wiley
,
Hoboken
,
NJ
,
2016
).
47.
A.
Slav
 et al,
ACS Appl. Nano Mater.
2
,
3626
(
2019
).
48.
J.
Kosmaca
,
R.
Meija
,
M.
Antsov
,
G.
Kunakova
,
R.
Sondors
,
I.
Iatsunskyi
,
E.
Coy
,
J.
Doherty
,
S.
Biswas
,
J. D.
Holmes
, and
D.
Erts
,
Nanoscale
11
,
13612
(
2019
).
49.
B.
Boote
,
L.
Men
,
H.
Andaraarachchi
,
U.
Bhattacharjee
,
J. W.
Petrich
,
J.
Vela
, and
E.
Smith
,
Chem. Mat.
29
,
6012
(
2017
).