23 research outputs found
Growth of germanium-silver surface alloys followed by in situ scanning tunneling microscopy: Absence of germanene formation
Theoretical studies have shown that new physical properties such as tunable
gap openings or quantum spinHall effects could be expected from group-IV
graphene analogs (silicene, germanene, stanene). While therehave been numerous
studies of growth of such Si, Ge, Sn monolayers, the demonstration of their
hexagonalorganization has been often based on postgrowth characterization, and
their analogy to graphene has remainedcontroversial. Our real-time scanning
tunneling microscopy (STM) observation during Ge deposition on Ag(111)in the
380--430 K temperature range reveals that Ag atoms are involved in all the
structures observed beforethe formation of a second layer, rejecting the
possible formation of germanene on this substrate within theseexperimental
conditions. The observation by STM of Ge atomic diffusion shows that easy
exchange between Agand Ge atoms is responsible for the Ge-Ag surface alloying
at such temperatures
Transport and Phototransport in ITO Nanocrystals with Short to Long-Wave Infrared Absorption
Nanocrystals are often described as an interesting strategy for the design of
low-cost optoelectronic devices especially in the infrared range. However the
driving materials reaching infrared absorption are generally heavy
metalcontaining (Pb and Hg) with a high toxicity. An alternative strategy to
achieve infrared transition is the use of doped semiconductors presenting
intraband or plasmonic transition in the short, mid and long-wave infrared.
This strategy may offer more flexibility regarding the range of possible
candidate materials. In particular, significant progresses have been achieved
for the synthesis of doped oxides and for the control of their doping
magnitude. Among them, tin doped indium oxide (ITO) is the one providing the
broadest spectral tunability. Here we test the potential of such ITO
nanoparticles for photoconduction in the infrared. We demonstrate that In2O3
nanoparticles presents an intraband absorption in the mid infrared range which
is transformed into a plasmonic feature as doping is introduced. We have
determined the cross section associated with the plasmonic transition to be in
the 1-3x10-13 cm2 range. We have observed that the nanocrystals can be made
conductive and photoconductive due to a ligand exchange using a short
carboxylic acid, leading to a dark conduction with n-type character. We bring
further evidence that the observed photoresponse in the infrared is the result
of a bolometric effect
How far the Chemistry of the Self-Assembly Monolayers on Gold Surfaces Affects their Work Function?
International audienceSelf-assembled monolayers composed of various long-chain aliphatic molecules and different tail functional groups have been synthesized on the Au(111) surface and characterized by Kelvin Probe Force Microscopy and Ultraviolet Photoelectron Spectroscopy. Carboxy, amino, thio and methyl terminal groups have been considered in the design of self-assembled monolayers with different aliphatic chain lengths (from C6 to C16). The work function measurements by Kelvin Probe Force Microscopy have been carried out under controled and room atmosphere. Remarkably, a reduction of the relative humidity from 40 % to 3 % has induced a work function shift up to 0.3 eV. As expected, the changes of the chain length of the aliphatic moiety and of the tail group have a significant impact on the tuning of the measured work function (3.90 eV for dodecanethiol versus 4.57 eV for mercaptohexadecylamine). Surprisingly, the change of the net dipole moment of the tail group (sign and amplitude) does not dominate the work function variations. In contrast, the change of the chain length and the possibility of the tail group to form a complex hydrogen-bond network between molecules, lead to significant modulations of the work function. In order to interpret these original findings, density functional theory models of equivalent self-assembled monolayers adsorbed on the Au(111) surface have been developed at an unprecedented level of description with large supercells including simultaneously 27 coadsorbed molecules and weak van der Waals interactions between them. Such large systems have allowed the theoretical modeling of complex hydrogen-bond networks between molecules when possible (carboxy tail group). The comparison between computed and measured work functions shows a striking agreement, thus allowing the disentanglement of the previously mentioned competing effects. This consistency between experiment and theory will help designing the electronic properties of self-assembled monolayers in the context of molecular electronics and organic transistors
Intraband transition in self-doped narrow band gap colloidal quantum dots
International audienceIn this article we discuss the infrared properties of self-doped nanocrystals and in particular the case of HgSe. HgSe colloidal quantum dots have recently been reported for their tunable optical features all over the mid infrared from 3 to 20 ÎŒm. Their optical absorption is a combination of interband absorption at high energy and intraband absorption at low energy. The latter results from the self-doped character of HgSe. The origin of this self-doping is also discussed. We demonstrated that the doping results from the combination of the narrow band gap and high work function of HgSe, which leads to a reduction of the CQD by the water in the environment. In addition, we demonstrated that the doping density can be tuned over an order of magnitude thanks to the control of the capping ligands
High Tolerance of Double-Decker Phthalocyanine Towards Molecular Oxygen
Because organic electronics
suffer from degradation-inducing oxidation processes, oxygen-tolerant
organic molecules could solve this issue and be integrated to improve
the stability of devices during operation. In this work, we investigate
how lutetium double-decker phthalocyanine (LuPc2) reacts
toward molecular oxygen and we report microscopic details of its interaction
with LuPc2 film by combining X-ray photoemission spectroscopy,
near-edge X-ray absorption fine structure spectroscopy, and density
functional theory. Surprisingly, LuPc2 molecules are found
to weakly physisorb below 120 K and appear rather inert to molecular
oxygen at more elevated temperatures. We are able to draw a microscopic
picture at low temperature, in which oxygen molecules stick on top
of the pyrrolic carbon of LuPc2. Our work sheds light on
a class of semiconducting molecules, namely, double-decker phthalocyanines,
which present a high tolerance toward molecular oxygen, opening promising
perspectives for the design of stable materials to be applied in the
next generation of organic-based electronic devices operating under
ambient conditions
Electronic structure of CdSe-ZnS 2D nanoplatelets
International audienceAmong colloidal nanocrystals, 2D nanoplatelets (NPLs) made of cadmium chalcogenides have led to especially well controlled optical features. However, the growth of core shell heterostructures has so far been mostly focused on CdS shells, while more confined materials will be more promising to decouple the emitting quantum states of the core from their external environment. Using k·p simulation, we demonstrate that a ZnS shell reduces by a factor 10 the leakage of the wavefunction into the surrounding medium. Using X-ray photoemission (XPS), we confirm that the CdSe active layer is indeed unoxidized. Finally, we build an effective electronic spectrum for these CdSe/ZnS NPLs on an absolute energy scale which is a critical set of parameters for the future integration of this material into optoelectronic devices. We determine the work function (WF) to be 4.47âeV while the material is behaving as an n-type semiconductor
Size and catalytic activity of supported gold nanoparticles: an in operando study during CO oxidation
International audienceThe origin of the catalytic activity of gold nanoparticles remains debated despite extensive studies. This in operando work investigates the relationship between catalytic activity and size/shape of gold nanoparticles supported on TiO2(110) during CO oxidation. The nanoparticles were synthesized by vapor deposition in ultrahigh vacuum. Their geometry was monitored in the presence of O2, Ar, or a mixture of O2 + CO and of Ar + CO by grazing incidence small-angle X-ray scattering simultaneously with the catalytic activity. The occurrence of CO oxidation induces a sintering directly correlated to the reaction rate. The catalytic activity is optimum for a nanoparticle's diameter of 2.1 +/- 0.3 nm and a height of about six atomic layers. Below this size, the activity drop corresponds to a height decrease. Rescaling of activities obtained in different experimental conditions shows consistency of these results with published data using both "model" and "real" catalysts
Impact of dimensionality and confinement on the electronic properties of mercury chalcogenide nanocrystals
International audienceWe demonstrate the growth of 2D nanoplatelets (NPLs) made of a HgTe/CdS heterostructure, with an optical absorption reaching the shortwave infrared range. The material is an interesting platform to investigate the effect of dimensionality (0D vs 2D) and confinement on the electronic spectrum and carrier dynamics in colloidal materials. We bring consistent evidence for the p-type nature of this material from transport and photoemission measurements. The majority carrier dynamics probed using pump-probe photoemission is found to be mostly dependent on the presence of a confinement barrier at the surface rather than on the material dimensionality. The minority carrier, on the other hand, is strongly affected by the material shape showing longer lived minority carrier in 2D NPLs compared to their 0D equivalent with similar band gap. Finally, we test the potential of this material for photodetection in the short-wave infrared range (SWIR) and show that fast photoresponse and detectivity reaching 109 Jones at room temperature can be achieved
Intraband Mid-Infrared Transitions in Ag 2 Se Nanocrystals: Potential and Limitations for Hgfree Low Cost Photodetection
International audienceInfrared photodetection based on colloidal nanoparticles is a promising path toward low cost devices. However, mid-infrared absorption relies on interband transition in heavy metal based materials, which is a major flaw for the development toward mass market. In the quest of infrared active colloidal materials, we here investigate Ag2Se nanoparticles presenting intraband transition between 3 and 15 ”m. With photoemission and infrared spectroscopy, we are able to propose an electronic spectrum of the material in absolute energy scale. We also investigate the origin of doping and demonstrate that it is the result of cation excess under Ag+ form. We demonstrate photoconduction into this material including under resonant excitation of the intraband transition. However, performances are currently quite weak with (i) a slow photoresponse (several seconds), and (ii) some electrochemical instabilities at room temperature