Initial stage of MBE growth of MoSe2 monolayer.

An atomically thin MoSe2 layer has been synthesized on mica using molecular beam epitaxy (MBE). The polymorphous of the MoSe2 layer depends on the coverage and the growth temperature. At low coverages and low growth temperature, 1T-MoSe2 forms in addition to a comparable quantity of 2H-MoSe2. The metastable 1T-MoSe2 transfers gradually to the stable 2H-MoSe2 before the completion of the first monolayer. The current result sheds some light on the complexity of the nucleation and growth of transition metal dichalcogenide (TMDC) monolayers and implies a possible route for a phase selective synthesis using MBE.

Real-time monitoring of 2D semiconductor film growth with optical spectroscopy.

Real-time monitoring of the growth is essential for synthesizing high quality two dimensional (2D) transition-metal dichalcogenides with precisely controlled thickness. Here, we report the first real time in situ optical spectroscopic study on the molecular beam epitaxy of atomically thin molybdenum diselenide (MoSe2) films on sapphire substrates using differential reflectance spectroscopy. The characteristic optical spectrum of MoSe2 monolayer is clearly distinct from that of bilayer allowing a precise control of the film thickness during the growth. Furthermore, the evolution of the characteristic differential reflectance spectrum of the MoSe2 thin film as a function of the thickness sheds light on the details of the growth process. Our result demonstrates the importance and the great potential of the real time in situ optical spectroscopy for the realization of controlled growth of 2D semiconductor materials.

Layer resolved evolution of the optical properties of α-sexithiophene thin films.

We report a combined reflectance difference spectroscopy and scanning tunneling microscopy study of ultrathin α-sexithiophene (6T) films deposited on the Cu(110)-(2×1)O surface. The correlation between the layer resolved crystalline structure and the corresponding optical spectra data reveals a highly sensitive dependence of the excitonic optical properties on the layer thickness and crystalline structure of the 6T film.

Monitoring preparation and phase transitions of carburized W(1 1 0) by reflectance difference spectroscopy.

Reflectance difference spectroscopy (RDS) is applied to follow in situ the preparation of clean and carburized W(1 1 0) surfaces and to study the temperature-induced transition between the R(15 × 3) and R(15 × 12) carbon/tungsten surface phases. RDS data for this transition are compared to data obtained from Auger-electron spectroscopy and low-energy electron diffraction. All techniques reveal that this transition, occurring around 1870 K, is reversible with a small hysteresis, indicating a first-order-like behaviour. The present results also prove a high surface sensitivity of RDS, which is attributed to the excitation of electronic p-like surface resonances of W(1 1 0).

Stranski-Krastanov growth of para-sexiphenyl on Cu(110)-(2×1)O revealed by optical spectroscopy.

We have studied the growth of para-sexiphenyl (p-6P) on the Cu(110)-(2×1)O surface using reflectance difference spectroscopy (RDS) in combination with scanning tunneling microscopy (STM). The evolution of the optical anisotropy reveals that the growth of p-6P on the Cu(110)-(2×1)O surface at room temperature follows the Stranski-Krastanov growth mode with a two monolayer thick wetting layer. During all stages of growth, the p-6P molecules are well orientated with their long molecular axis aligned parallel to the Cu-O rows along the [001] direction of the Cu(110) substrate. The high packing density of the p-6P molecules in the first and second monolayer evidenced by RDS and STM is believed to be responsible for the switch from layer-by-layer to three-dimensional island growth.

Revealing the buried interface: para-sexiphenyl thin films grown on TiO2(110).

The thickness dependent optical and electronic structure of para-sexiphenyl thin films grown on TiO(2)(110) at around 400 K reveals that the substrate is first wet by one monolayer of molecules lying with their long axis parallel to the [001] direction of the substrate, while the molecules in subsequent layers are almost standing upright. Whilst ultraviolet photoemission spectroscopy (UPS) is sensitive to the molecules in the outermost layer, reflection difference spectroscopy (RDS) shows that the molecules at the buried interface do not dewet and maintain the orientation of the original wetting monolayer.

Growth oscillation of MoSe2 monolayers observed by differential reflectance spectroscopy.

The precisely controlled growth of transition metal dichalcogenide (TMDC) monolayers requires sensitive and nondestructive techniques to monitor the morphology and coverage in situ and in real time. In the current work, differential reflectance spectroscopy (DRS) was applied to monitor the molecular beam epitaxy (MBE) growth of atomically thin MoSe2 layers on mica. The optical evolution exhibits an oscillation with monolayer periodicity, revealing a two-dimensional (2D) layer-by-layer growth of the MoSe2 thin films. The observed sensitivity of DRS to the step density is associated to the modified electronic structures at the edges of TMDC monolayers. As DRS works in any transparent ambient, we speculate that it could be of great use for realizing precisely controlled growth of TMDC monolayers using not only MBE but also chemical vapor deposition (CVD).

Direct observation of the CVD growth of monolayer MoS2 using in situ optical spectroscopy.

Real-time monitoring is essential for understanding and precisely controlling of growth of two-dimensional transition metal dichalcogenide (2D TMDC) materials. However, it is very challenging to carry out such studies during chemical vapor deposition (CVD). Here, we report the first, real time, in situ study of the CVD growth of 2D TMDCs. More specifically, the CVD growth of a molybdenum disulfide (MoS2) monolayer on sapphire substrates has been monitored in situ using differential transmittance spectroscopy (DTS). The growth of the MoS2 monolayer can be precisely followed by observation of the evolution of the characteristic optical features. Consequently, a strong correlation between the growth rate of the MoS2 monolayer and the temperature distribution in the CVD reactor has been revealed. Our results demonstrate the great potential of real time, in situ optical spectroscopy to assist the precisely controlled growth of 2D semiconductor materials.

Probing optical excitations in chevron-like armchair graphene nanoribbons.

The bottom-up fabrication of graphene nanoribbons (GNRs) has opened new opportunities to specifically tune their electronic and optical properties by precisely controlling their atomic structure. Here, we address excitation in GNRs with periodic structural wiggles, the so-called chevron GNRs. Based on reflectance difference and high-resolution electron energy loss spectroscopies together with ab initio simulations, we demonstrate that their excited-state properties are of excitonic nature. The spectral fingerprints corresponding to different reaction stages in their bottom-up fabrication are also unequivocally identified, allowing us to follow the exciton build-up from the starting monomer precursor to the final GNR structure.

Exciton-dominated optical response of ultra-narrow graphene nanoribbons.

Narrow graphene nanoribbons exhibit substantial electronic bandgaps and optical properties fundamentally different from those of graphene. Unlike graphene--which shows a wavelength-independent absorbance for visible light--the electronic bandgap, and therefore the optical response, of graphene nanoribbons changes with ribbon width. Here we report on the optical properties of armchair graphene nanoribbons of width N=7 grown on metal surfaces. Reflectance difference spectroscopy in combination with ab initio calculations show that ultranarrow graphene nanoribbons have fully anisotropic optical properties dominated by excitonic effects that sensitively depend on the exact atomic structure. For N=7 armchair graphene nanoribbons, the optical response is dominated by absorption features at 2.1, 2.3 and 4.2 eV, in excellent agreement with ab initio calculations, which also reveal an absorbance of more than twice the one of graphene for linearly polarized light in the visible range of wavelengths.

Dense passivating poly(ethylene glycol) films on indium tin oxide substrates.

We describe the formation and characterization of surface-passivating poly(ethylene glycol) (PEG) films on indium tin oxide (ITO) glass substrates. PEG chains with a molecular weight of 2000 and 5000 D were covalently attached to the substrates in a systematic approach using different coupling schemes. The coupling strategies included the direct grafting with PEG-silane, PEG-methacrylate, and PEG-bis(amine), as well as the two-step functionalization with aldehyde-bearing silane films and subsequent coupling with PEG-bis(amine). Elemental analysis by X-ray photoelectron spectroscopy (XPS) confirmed the successful surface modification, and XPS and ellipsometry provided values for film thicknesses. XPS and ellipsometry thickness values were almost identical for PEG-silane films but differed by up to 400% for the other PEG layers, suggesting a homogeneous layer for PEG-silane but an inhomogeneous distribution for other PEG coatings on the molecularly rough ITO substrates. Atomic force microscopy (AFM) and water contact angle goniometry confirmed the different degrees of surface homogeneity of the polymer films, with PEG-silane reducing the AFM rms surface roughness by 50% and the water contact angle hysteresis by 75% compared to uncoated ITO. The ability of the PEG layers to passivate the substrate against the nonspecific adsorption of biopolymers was tested using fluorescence-labeled immunoglobulin G and DNA oligonucleotides in combination with fluorescence microscopy. The results indicate a positive relationship between film density and homogeneity on one hand and the ability to passivate against biopolymer adhesion on the other hand. The most homogeneous layers prepared with PEG-silane reduced the nonspecific adsorption of fluorescence-labeled DNA by a factor of 300 compared to uncoated ITO. In addition, the study finds that the ratio of film thicknesses derived by ellipsometry and XPS is a useful parameter to quantify the structural integrity of PEG layers on molecularly rough ITO surfaces. The findings may be applied to characterize PEG or other polymeric films on similarly coarse substrates.

Preparation and characterization of dense films of poly(amidoamine) dendrimers on indium tin oxide.

Indium tin oxide (ITO) substrates were modified with a layer of poly(amidoamine) (PAMAM) dendrimers to change their surface properties and, in particular, the substrates' work function. The functionalization procedure involved the electrostatic adsorption of positively charged PAMAM dendrimers of generation five onto negatively polarized ITO surfaces. Three different PAMAM dendrimers were used: PAMAM-NH2 and PAMAM-OH with terminal amine and hydroxyl groups, respectively, as well as Q-PAMAM-NH2, which had been prepared from PAMAM-NH2 by quaternization of the dendrimer's terminal and internal amine groups with methyl iodide. The resulting organic films were analyzed by contact angle goniometry, X-ray photoelectron spectroscopy, ellipsometry, and Kelvin probe force microscopy to confirm the presence of a dense layer. A Langmuir isotherm was derived from surface densities of fluorescence-labeled PAMAM-NH2 dendrimers from which we deduced an equilibrium binding constant, K(eq), of (1.3 +/- 0.3) x 10(5) M(-1). Kelvin probe measurements of the contact potential difference revealed a high reduction of the work function from 4.9 eV for bare ITO to 4.3 eV for ITO with a dense film of PAMAM-NH2 of generation five. PAMAM-OH and Q-PAMAM-NH2 resulted in slightly smaller work function changes. This study illustrates that the work function of ITO can be tuned by adlayers composed of PAMAM dendrimers.