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1.
J Phys Chem C Nanomater Interfaces ; 126(27): 11400-11406, 2022 Jul 14.
Artigo em Inglês | MEDLINE | ID: mdl-35865793

RESUMO

Germanium sulfide (GeS) is a layered monochalcogenide semiconductor with a band gap of about 1.6 eV. To verify the suitability of GeS for field-effect-based device applications, a detailed understanding of the electronic transport mechanisms of GeS-metal junctions is required. In this work, we have used conductive atomic force microscopy (c-AFM) to study charge carrier injection in metal-GeS nanocontacts. Using contact current-voltage spectroscopy, we identified three dominant charge carrier injection mechanisms: thermionic emission, direct tunneling, and Fowler-Nordheim tunneling. In the forward-bias regime, thermionic emission is the dominating current injection mechanism, whereas in the reverse-bias regime, the current injection mechanism is quantum mechanical tunneling. Using tips of different materials (platinum, n-type-doped silicon, and highly doped p-type diamond), we found that the Schottky barrier is almost independent of the work function of the metallic tip, which is indicative of a strong Fermi-level pinning. This strong Fermi-level pinning is caused by charged defects and impurities.

2.
Nat Commun ; 12(1): 2542, 2021 May 05.
Artigo em Inglês | MEDLINE | ID: mdl-33953174

RESUMO

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates.

3.
ACS Nano ; 15(4): 7421-7429, 2021 Apr 27.
Artigo em Inglês | MEDLINE | ID: mdl-33759515

RESUMO

Like other 2D materials, the boron-based borophene exhibits interesting structural and electronic properties. While borophene is typically prepared by molecular beam epitaxy, we report here on an alternative way of synthesizing large single-phase borophene domains by segregation-enhanced epitaxy. X-ray photoelectron spectroscopy shows that borazine dosing at 1100 °C onto Ir(111) yields a boron-rich surface without traces of nitrogen. At high temperatures, the borazine thermally decomposes, nitrogen desorbs, and boron diffuses into the substrate. Using time-of-flight secondary ion mass spectrometry, we show that during cooldown the subsurface boron segregates back to the surface where it forms borophene. In this case, electron diffraction reveals a (6 × 2) reconstructed borophene χ6-polymorph, and scanning tunneling spectroscopy suggests a Dirac-like behavior. Studying the kinetics of borophene formation in low energy electron microscopy shows that surface steps are bunched during the borophene formation, resulting in elongated and extended borophene domains with exceptional structural order.

4.
ACS Nano ; 14(10): 13629-13637, 2020 Oct 27.
Artigo em Inglês | MEDLINE | ID: mdl-32910634

RESUMO

Cluster superlattice membranes consist of a two-dimensional hexagonal lattice of similar-sized nanoclusters sandwiched between single-crystal graphene and an amorphous carbon matrix. The fabrication process involves three main steps, the templated self-organization of a metal cluster superlattice on epitaxial graphene on Ir(111), conformal embedding in an amorphous carbon matrix, and subsequent lift-off from the Ir(111) substrate. The mechanical stability provided by the carbon-graphene matrix makes the membrane stable as a free-standing material and enables transfer to other substrates. The fabrication procedure can be applied to a wide variety of cluster materials and cluster sizes from the single-atom limit to clusters of a few hundred atoms, as well as other two-dimensional layer/host matrix combinations. The versatility of the membrane composition, its mechanical stability, and the simplicity of the transfer procedure make cluster superlattice membranes a promising material in catalysis, magnetism, energy conversion, and optoelectronics.

5.
ACS Appl Mater Interfaces ; 11(43): 40524-40532, 2019 Oct 30.
Artigo em Inglês | MEDLINE | ID: mdl-31588723

RESUMO

Iridium cluster superlattices on the graphene moiré with Ir(111) are embedded with elemental carbon through vapor-phase deposition. Using scanning tunneling microscopy and spectroscopy, we find that carbon embedding is conformal and does not deteriorate the excellent order of the iridium clusters. The thermal and mechanical stability of the embedded clusters is greatly enhanced. Smoluchowski ripening as well as cluster pick-up by the scanning tunneling microscopy tip are both suppressed. The only cluster decay path left takes place at an elevated temperature of around 1050 K. The cluster material penetrates through the graphene sheet, whereby it becomes bound to the underlying metal. It is argued that conformal carbon embedding is an important step towards the formation of a new type of sintering-resistant cluster lattice material for nanocatalysis and nanomagnetism.

6.
J Phys Chem Lett ; 10(10): 2578-2584, 2019 May 16.
Artigo em Inglês | MEDLINE | ID: mdl-31041866

RESUMO

Here we report on a novel, noninvasive route for operando tailoring of the charge transport properties of metal/WS2 contacts without the negative impacts to two-dimensional materials arising from conventional doping methods. The doping level of thin WS2 flakes supported on insulating mica is susceptible to local charge variations induced by the presence of a hydration layer between mica and WS2. We demonstrate, via the use of several complementary scanning probe techniques, that the direct control of the state and thickness of this intercalated water film controls the charge injection properties of Pt/WS2 nanocontacts. A switch from unipolar to ambipolar transport was achieved by environmentally controlling the thickness of the intercalated water. We show that the effect persists even for multilayer flakes and that it is completely reversible, opening a new route toward the realization of novel electronics with environmentally controllable functionalities.

7.
Beilstein J Nanotechnol ; 10: 696-705, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-30931211

RESUMO

The formation of self-assembled superstructures of cetyltrimethylammonium bromide (CTAB) after drying on a nonwetting highly ordered pyrolytic graphite (HOPG) surface have been investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Although SEM did not reveal coverage of CTAB layers, AFM showed not only CTAB assembly, but also the dynamics of the process on the surface. The self-assembled layers of CTAB molecules on the HOPG terraces prior to nanorod deposition were shown to change the wettability of the surface, and as a result, gold nanorod deposition takes place on nonwetting HOPG terraces.

8.
J Phys Chem C Nanomater Interfaces ; 123(9): 5411-5420, 2019 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-30873255

RESUMO

Understanding the electron transport through transition-metal dichalcogenide (TMDC)-based semiconductor/metal junctions is vital for the realization of future TMDC-based (opto-)electronic devices. Despite the bonding in TMDCs being largely constrained within the layers, strong Fermi-level pinning (FLP) was observed in TMDC-based devices, reducing the tunability of the Schottky barrier height. We present evidence that metal-induced gap states (MIGS) are the origin for the large FLP similar to conventional semiconductors. A variety of TMDCs (MoSe2, WSe2, WS2, and MoTe2) were investigated using high-spatial-resolution surface characterization techniques, permitting us to distinguish between defected and pristine regions. The Schottky barrier heights on the pristine regions can be explained by MIGS, inducing partial FLP. The FLP strength is further enhanced by disorder-induced gap states induced by transition-metal vacancies or substitutionals at the defected regions. Our findings emphasize the importance of defects on the electron transport properties in TMDC-based devices and confirm the origin of FLP in TMDC-based metal/semiconductor junctions.

9.
Nanoscale ; 11(12): 5548-5556, 2019 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-30860526

RESUMO

Transition metal dichalcogenides (TMDC) have recently attracted much attention as a promising platform for the realization of 2-dimensional (2D) electronic devices. One of the major challenges for their wide-scale application is the control of the potential barrier at the metal/TMDC junction. Using conductive atomic force microscopy (c-AFM) we have investigated modifications of the Schottky barrier height (SBH) across a Pt/WS2 junction by the introduction of thin buffer layers of graphene and MoSe2. While graphene greatly reduces the contact resistance in both bias directions, thin layers of MoSe2 lower the Schottky barrier and leave the rectifying properties of the junction intact. We have studied the dependence of the transport properties on the thickness of the graphene and MoSe2 buffer layers. In both cases, the charge transport characteristics can be tailored by varying the buffer layer thickness. The edge of single layer graphene is observed to form an ohmic contact to the underlying WSe2 substrate. This study demonstrates that the introduction of atomically thin MoSe2 and graphene buffer layers is a feasible and elegant method to control the Schottky barrier when contacting TMDCs. The results are important for the fabrication of devices utilizing 2D materials.

10.
Langmuir ; 35(14): 4886-4892, 2019 Apr 09.
Artigo em Inglês | MEDLINE | ID: mdl-30883130

RESUMO

The friction of graphene on mica was studied using lateral force microscopy. We observed that intercalation of alcohol molecules significantly increases the friction of graphene, as compared to water. An increase of 1.8, 2.4, and 5.9 times in friction between the atomic force microscopy tip and  single-layer graphene was observed for methanol, ethanol, and 2-propanol, respectively. Moreover, the friction of graphene is found to be higher for single-layer graphene than for multilayer graphene. We attribute the increase in friction to the additional vibrational modes of alcohol molecules. The significant variation of the frictional characteristics of graphene at the nanoscale by altering the intercalant could open up applications for the next-generation nanolubricants and nanodevices.

11.
Nano Lett ; 19(2): 1190-1196, 2019 02 13.
Artigo em Inglês | MEDLINE | ID: mdl-30582818

RESUMO

The down-scaling of electrical components requires a proper understanding of the physical mechanisms governing charge transport. Here, we have investigated atomic-scale contacts and their transport characteristics on WS2 using conductive atomic force microscopy (c-AFM). We demonstrate that c-AFM can provide true atomic resolution, revealing atom vacancies, adatoms, and periodic modulations arising from electronic effects. Moreover, we find a lateral variation of the surface conductivity that arises from the lattice periodicity of WS2. Three distinct sites are identified, i.e., atop, bridge, and hollow. The current transport across these atomic metal-semiconductor interfaces is understood by considering thermionic emission and Fowler-Nordheim tunnelling. Current modulations arising from point defects and the contact geometry promise a novel route for the direct control of atomic point contacts in diodes and devices.

12.
J Phys Chem C Nanomater Interfaces ; 122(44): 25498-25505, 2018 Nov 08.
Artigo em Inglês | MEDLINE | ID: mdl-30450151

RESUMO

HfSe2 is a very good candidate for a transition metal dichalcogenide-based field-effect transistor owing to its moderate band gap of about 1 eV and its high-κ dielectric native oxide. Unfortunately, the experimentally determined charge carrier mobility is about 3 orders of magnitude lower than the theoretically predicted value. This strong deviation calls for a detailed investigation of the physical and electronic properties of HfSe2. Here, we have studied the structure, density, and density of states of several types of defects that are abundant on the HfSe2 surface using scanning tunneling microscopy and spectroscopy. Compared to MoS2 and WSe2, HfSe2 exhibits similar type of defects, albeit with a substantially higher density of 9 × 1011 cm-2. The most abundant defect is a subsurface defect, which shows up as a dim feature in scanning tunneling microscopy images. These dim dark defects have a substantially larger band gap (1.25 eV) than the pristine surface (1 eV), suggesting a substitution of the Hf atom by another atom. The high density of defects on the HfSe2 surface leads to very low Schottky barrier heights. Conductive atomic force microscopy measurements reveal a very small dependence of the Schottky barrier height on the work function of the metals, suggesting a strong Fermi-level pinning. We attribute the observed Fermi-level pinning (pinning factor ∼0.1) to surface distortions and Se/Hf defects. In addition, we have also studied the HfSe2 surface after the exposure to air by scanning tunneling microscopy and conductive atomic force microscopy. Partly oxidized layers with band gaps of 2 eV and Schottky barrier heights of ∼0.6 eV were readily found on the surface. Our experiments reveal that HfSe2 is very air-sensitive, implying that capping or encapsulating of HfSe2, in order to protect it against oxidation, is a necessity for technological applications.

13.
Nat Commun ; 9(1): 4426, 2018 10 24.
Artigo em Inglês | MEDLINE | ID: mdl-30356162

RESUMO

High quality graphene nanoribbons epitaxially grown on the sidewalls of silicon carbide (SiC) mesa structures stand as key building blocks for graphene-based nanoelectronics. Such ribbons display 1D single-channel ballistic transport at room temperature with exceptionally long mean free paths. Here, using spatially-resolved two-point probe (2PP) measurements, we selectively access and directly image a range of individual transport modes in sidewall ribbons. The signature of the independently contacted channels is a sequence of quantised conductance plateaus for different probe positions. These result from an interplay between edge magnetism and asymmetric terminations at opposite ribbon edges due to the underlying SiC structure morphology. Our findings demonstrate a precise control of transport through multiple, independent, ballistic tracks in graphene-based devices, opening intriguing pathways for quantum information device concepts.

14.
ACS Appl Mater Interfaces ; 10(15): 13218-13225, 2018 Apr 18.
Artigo em Inglês | MEDLINE | ID: mdl-29578328

RESUMO

Here, we report on the surface conductivity of WSe2 and Mo xW1- xSe2 (0 ≤ x ≤ 1) crystals investigated with conductive atomic force microscopy. We found that stacking faults, defects, and chemical heterogeneities form distinct two-dimensional and one-dimensional conduction paths on the transition metal dichalcogenide surface. In the case of WSe2, in addition to step edges, we find a significant amount of stacking faults (formed during the cleaving process) that strongly influence the surface conductivity. These regions are attributed to the alternation of the 2H and 3R polytypism. The stacking faults form regular 2D patterns by alternation of the underlying stacking order, with a periodicity that varies significantly between different regions and samples. In the case of Mo xW1- xSe2, its conductivity has a localized nature, which depends on the underlying chemical composition and the Mo/W ratio. Segregation to W-rich and Mo-rich regions during the growth process leads to nonuniform conduction paths on the surface of the alloy. We found a gradual change of the conductivity moving from one region to the other, reminiscent of lateral band bending. Our results demonstrate the use of C-AFM as a nanoscopic tool to probe the electrical properties of largely inhomogeneous samples and show the complicated nature of the surface conductivity of TMDC alloys.

15.
ACS Nano ; 11(12): 12723-12731, 2017 12 26.
Artigo em Inglês | MEDLINE | ID: mdl-29112376

RESUMO

The classic regelation experiment of Thomson in the 1850s deals with cutting an ice cube, followed by refreezing. The cutting was attributed to pressure-induced melting but has been challenged continuously, and only lately consensus emerged by understanding that compression shortens the O:H nonbond and lengthens the H-O bond simultaneously. This H-O elongation leads to energy loss and lowers the melting point. The hot debate survived well over 150 years, mainly due to a poorly defined heat exchange with the environment in the experiment. In our current experiment, we achieved thermal isolation from the environment and studied the fully reversible ice-liquid water transition for water confined between graphene and muscovite mica. We observe a transition from two-dimensional (2D) ice into a quasi-liquid phase by applying a pressure exerted by an atomic force microscopy tip. At room temperature, the critical pressure amounts to about 6 GPa. The transition is completely reversible: refreezing occurs when the applied pressure is lifted. The critical pressure to melt the 2D ice decreases with temperature, and we measured the phase coexistence line between 293 and 333 K. From a Clausius-Clapeyron analysis, we determine the latent heat of fusion of two-dimensional ice at 0.15 eV/molecule, being twice as large as that of bulk ice.

16.
Langmuir ; 33(43): 11977-11985, 2017 10 31.
Artigo em Inglês | MEDLINE | ID: mdl-28985466

RESUMO

We use atomic force microscopy to in situ investigate the dynamic behavior of confined water at the interface between graphene and mica. The graphene is either uncharged, negatively charged, or positively charged. At high humidity, a third water layer will intercalate between graphene and mica. When graphene is negatively charged, the interface fills faster with a complete three layer water film, compared to uncharged graphene. As charged positively, the third water layer dewets the interface, either by evaporation into the ambient or by the formation of three-dimensional droplets under the graphene, on top of the bilayer. Our experimental findings reveal novel phenomena of water at the nanoscale, which are interesting from a fundamental point of view and demonstrate the direct control over the wetting properties of the graphene/water interface.

17.
J Colloid Interface Sci ; 505: 776-782, 2017 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-28666222

RESUMO

Direct growth of flat micrometer-sized bilayer graphene islands in between molybdenum disulfide sheets is achieved by chemical vapor deposition of ethylene at about 800°C. The temperature assisted decomposition of ethylene takes place mainly at molybdenum disulfide step edges. The carbon atoms intercalate at this high temperature, and during the deposition process, through defects of the molybdenum disulfide surface such as steps and wrinkles. Post growth atomic force microscopy images reveal that circular flat graphene islands have grown at a high yield. They consist of two graphene layers stacked on top of each other with a total thickness of 0.74nm. Our results demonstrate direct, simple and high yield growth of graphene/molybdenum disulfide heterostructures, which can be of high importance in future nanoelectronic and optoelectronic applications.

18.
ACS Appl Mater Interfaces ; 9(22): 19278-19286, 2017 Jun 07.
Artigo em Inglês | MEDLINE | ID: mdl-28508628

RESUMO

Understanding the electronic contact between molybdenum disulfide (MoS2) and metal electrodes is vital for the realization of future MoS2-based electronic devices. Natural MoS2 has the drawback of a high density of both metal and sulfur defects and impurities. We present evidence that subsurface metal-like defects with a density of ∼1011 cm-2 induce negative ionization of the outermost S atom complex. We investigate with high-spatial-resolution surface characterization techniques the effect of these defects on the local conductance of MoS2. Using metal nanocontacts (contact area < 6 nm2), we find that subsurface metal-like defects (and not S-vacancies) drastically decrease the metal/MoS2 Schottky barrier height as compared to that in the pristine regions. The magnitude of this decrease depends on the contact metal. The decrease of the Schottky barrier height is attributed to strong Fermi level pinning at the defects. Indeed, this is demonstrated in the measured pinning factor, which is equal to ∼0.1 at defect locations and ∼0.3 at pristine regions. Our findings are in good agreement with the theoretically predicted values. These defects provide low-resistance conduction paths in MoS2-based nanodevices and will play a prominent role as the device junction contact area decreases in size.

19.
Sci Rep ; 7: 43451, 2017 03 06.
Artigo em Inglês | MEDLINE | ID: mdl-28262710

RESUMO

The distribution of potassium (K+) ions on air-cleaved mica is important in many interfacial phenomena such as crystal growth, self-assembly and charge transfer on mica. However, due to experimental limitations to nondestructively probe single ions and ionic domains, their exact lateral organization is yet unknown. We show, by the use of graphene as an ultra-thin protective coating and scanning probe microscopies, that single potassium ions form ordered structures that are covered by an ice layer. The K+ ions prefer to minimize the number of nearest neighbour K+ ions by forming row-like structures as well as small domains. This trend is a result of repulsive ionic forces between adjacent ions, weakened due to screening by the surrounding water molecules. Using high resolution conductive atomic force microscopy maps, the local conductance of the graphene is measured, revealing a direct correlation between the K+ distribution and the structure of the ice layer. Our results shed light on the local distribution of ions on the air-cleaved mica, solving a long-standing enigma. They also provide a detailed understanding of charge transfer from the ionic domains towards graphene.

20.
Langmuir ; 32(26): 6582-90, 2016 07 05.
Artigo em Inglês | MEDLINE | ID: mdl-27275663

RESUMO

We present a new method to create dynamic nanobubbles. The nanobubbles are created between graphene and mica by reducing intercalated water to hydrogen. The nanobubbles have a typical radius of several hundred nanometers, a height of a few tens of nanometers and an internal pressure in the range of 0.5-8 MPa. Our approach paves the way to the realization of nanobubbles of which both size and internal pressure are tunable.

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