Abstract:The state of matter in fluid phases, determined by the interactions between particles, can be characterized by a pair correlation function (PCF). At the nanoscale, the PCF has been so far obtained experimentally only by means of reciprocal-space techniques. We use scanning tunneling microscopy (STM) at room temperature in combination with lattice-gas kinetic Monte Carlo (KMC) simulations to study a two-dimensional gas of highly mobile molecules of fluorinated copper phthalocyanine on a Si(111)/Tl-(1×1) surface… Show more
“…In fact, the 2D gas plays an indispensable role in the phase transformations, mediating mass transport between the individual islands. The presence of the 2D molecular gas is visible in our room-temperature STM images as characteristic noise caused by molecules passing below the scanning tip and rapid changes at the edges of the molecular islands. , …”
Section: Resultsmentioning
confidence: 84%
“…The presence of the 2D molecular gas is visible in our room-temperature STM images as characteristic noise caused by molecules passing below the scanning tip and rapid changes at the edges of the molecular islands. 31,32 The sample temperature was slowly increased at a rate of 3 K•min −1 from room temperature; at 341 K we observed the formation of β phase islands simultaneously at different locations within the LEEM view field (see Figures 4 and S5 and Supporting Videos SV1 and SV3). Altogether we make the following observations: (I) As the temperature is increased, before nucleation of the β phase islands, we observe a reduction in the area of the α phase islands (see Supporting Information, Section 5) as well as Ostwald ripening of the α phase (the small islands disappear and the larger ones grow).…”
Understanding
the nucleation and growth kinetics of thin films
is a prerequisite for their large-scale utilization in devices. For
self-assembled molecular phases near thermodynamic equilibrium the
nucleation–growth kinetic models are still not developed. Here,
we employ real-time low-energy electron microscopy (LEEM) to visualize
a phase transformation induced by the carboxylation of 4,4′-biphenyl
dicarboxylic acid on Ag(001) under ultra-high-vacuum conditions. The
initial (α) and transformed (β) molecular phases are characterized
in detail by X-ray photoemission spectroscopy, single-domain low-energy
electron diffraction, room-temperature scanning tunneling microscopy,
noncontact atomic force microscopy, and density functional theory
calculations. The phase transformation is shown to exhibit a rich
variety of phenomena, including Ostwald ripening of the α domains,
burst nucleation of the β domains outside the α phase,
remote dissolution of the α domains by nearby β domains,
and a structural change from disorder to order. We show that all phenomena
are well described by a general growth–conversion–growth
(GCG) model. Here, the two-dimensional gas of admolecules has a dual
role: it mediates mass transport between the molecular islands and
hosts a slow deprotonation reaction. Further, we conclude that burst
nucleation is consistent with a combination of rather weak intermolecular
bonding and the onset of an additional weak many-body attractive interaction
when a molecule is surrounded by its nearest neighbors. In addition,
we conclude that Ostwald ripening and remote dissolution are essentially
the same phenomenon, where a more stable structure grows at the expense
of a kinetically formed, less stable entity via transport
through the 2D gas. The proposed GCG model is validated through kinetic
Monte Carlo (kMC) simulations.
“…In fact, the 2D gas plays an indispensable role in the phase transformations, mediating mass transport between the individual islands. The presence of the 2D molecular gas is visible in our room-temperature STM images as characteristic noise caused by molecules passing below the scanning tip and rapid changes at the edges of the molecular islands. , …”
Section: Resultsmentioning
confidence: 84%
“…The presence of the 2D molecular gas is visible in our room-temperature STM images as characteristic noise caused by molecules passing below the scanning tip and rapid changes at the edges of the molecular islands. 31,32 The sample temperature was slowly increased at a rate of 3 K•min −1 from room temperature; at 341 K we observed the formation of β phase islands simultaneously at different locations within the LEEM view field (see Figures 4 and S5 and Supporting Videos SV1 and SV3). Altogether we make the following observations: (I) As the temperature is increased, before nucleation of the β phase islands, we observe a reduction in the area of the α phase islands (see Supporting Information, Section 5) as well as Ostwald ripening of the α phase (the small islands disappear and the larger ones grow).…”
Understanding
the nucleation and growth kinetics of thin films
is a prerequisite for their large-scale utilization in devices. For
self-assembled molecular phases near thermodynamic equilibrium the
nucleation–growth kinetic models are still not developed. Here,
we employ real-time low-energy electron microscopy (LEEM) to visualize
a phase transformation induced by the carboxylation of 4,4′-biphenyl
dicarboxylic acid on Ag(001) under ultra-high-vacuum conditions. The
initial (α) and transformed (β) molecular phases are characterized
in detail by X-ray photoemission spectroscopy, single-domain low-energy
electron diffraction, room-temperature scanning tunneling microscopy,
noncontact atomic force microscopy, and density functional theory
calculations. The phase transformation is shown to exhibit a rich
variety of phenomena, including Ostwald ripening of the α domains,
burst nucleation of the β domains outside the α phase,
remote dissolution of the α domains by nearby β domains,
and a structural change from disorder to order. We show that all phenomena
are well described by a general growth–conversion–growth
(GCG) model. Here, the two-dimensional gas of admolecules has a dual
role: it mediates mass transport between the molecular islands and
hosts a slow deprotonation reaction. Further, we conclude that burst
nucleation is consistent with a combination of rather weak intermolecular
bonding and the onset of an additional weak many-body attractive interaction
when a molecule is surrounded by its nearest neighbors. In addition,
we conclude that Ostwald ripening and remote dissolution are essentially
the same phenomenon, where a more stable structure grows at the expense
of a kinetically formed, less stable entity via transport
through the 2D gas. The proposed GCG model is validated through kinetic
Monte Carlo (kMC) simulations.
“…This is probably caused by a narrow temperature interval between formation of the 1×1 structure and Tl desorption 34 . We note that other less frequent types of defects coexisting with the ring-shaped ones can be observed occasionally 32 .Figure 1( a ) STM image of the Tl/Si(111)1×1 surface at sample voltage U S = +1 V showing ring-shaped defects. ( b ) Top and side views of the relaxed structure of the multi-vacancy defect on the Tl/Si(111)1×1 surface induced by the presence of an extra Si atom at the topmost atomic layer and missing Tl atoms.…”
Section: Resultsmentioning
confidence: 99%
“…Furthermore, we demonstrated the metallicity of the observed layers in contrast to predictions from density functional theory (DFT) calculations and discussed a possible doping effect of the defects. The defects also play an important role in the interaction with other adsorbates on the surface 32,33 . In the second important work, Sakamoto et al .…”
The Tl/Si(111)1 × 1 surface is a representative of a 2D layer with Rashba-type spin-split electronic bands. To utilize the spin polarization, doping of the system should be understood on atomic level. We present a study of two types of atomic defects predicted to dope the considered electronic system – Si-induced vacancies and defects associated with the presence of extra Tl atoms. Structural calculations based on density functional theory (DFT) confirm the stability of the proposed defect structure consisting of an extra Si atom and missing seven Tl atoms as proposed in an earlier experimental study. The calculated spatial charge distributions indicate an enhancement of the charge around the extra Si atom, which correctly reproduces topographies of the corresponding scanning tunneling microscopy images while the calculated local densities of states of this system explain obtained scanning tunneling spectra. The DFT structural calculations let us determine the atomic structure of the defect caused by the presence of an extra Tl atom. The calculated spatial charge distributions show a ring-like feature around the extra Tl atom. The obtained results indicate a charge transfer from the central extra Tl atom to its vicinity in the agreement with earlier photoemission measurements.
“…In our previous works we studied room-temperature behavior of F 0 CuPC and F 16 CuPC molecules deposited on the Si(111)-Tl 1 × 1 surface. We found that the F 0 CuPC molecules form ordered structures at full ML coverage.…”
Hydrogen-like
bonding between halogen and hydrogen atoms is widely
accepted as the driving force in self-ordering process of various
molecular layers. A proper understanding of how the molecular periphery
influences the self-ordering is crucial for formation of structurally
and electronically well-defined interfaces. To probe the stability
of ordered molecular layers of phthalocyanines with various fluorination
on the Si(111)-Tl 1 × 1 surface we introduce utilization of a
strong electric field of the tip of the scanning tunneling microscope.
Ab-initio calculations on extended lattices are used to distinguish
molecule–substrate and intermolecular interactions. A systematic
comparison of combinations of phthalocyanines with various fluorination
provides an understanding of how the molecular periphery influences
the intermolecular interaction. We demonstrate that the straightforward
explanation of the process of self-ordering by hydrogen-like bonding
is not acceptable in the studied case.
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