arXiv subjects

Ioan Baldea

Publications and source records attributed to Ioan Baldea.

At least 19 recordsLinked to original sources

arXiv2024

Dichotomy between Level Broadening and Level Coupling to Electrodes in Large Area EGaIn Molecular Junctions

Choosing self-assembled monolayers (SAM) of fluorine terminated oligophenylenes adsorbed on gold as illustration, we show that a single level (molecular orbital, MO) model can excellently reproduce full I-V curves measured for large area junctions fabricated with top EGaIn contact. In addition, this model unravels a surprising dichotomy between MO coupling to electrodes and the MO broadening. Importantly for the coherence of the microscopic description, the latter is found to correlate with the SAM coverage and molecular and pi* orbital tilt angles.

cond-mat.mes-hall↗

arXiv2024

Gaining insight into molecular tunnel junctions with a pocket calculator without I-V data fitting. Five-thirds protocol

The proposed protocol is an attempt to meet the experimentalists' legitimate desire of reliably and easily extracting microscopic parameters from current-voltage measurements on molecular junctions. It applies to junctions wherein charge transport dominated by a single level (molecular orbital, MO) occurs via off-resonant tunneling. The recipe is simple. The measured current-voltage curve $I = I(V)$ should be recast as a curve of $V^{5/3}/I$ versus $V$. This curve exhibits two maxima: one at positive bias ($V = V_{p+}$), another at negative bias ($V = V_{p-}$). The values $V_{p +} > 0$ and $V_{p -} < 0$ at the two peaks of the curve for $V^{5/3}/I$ at positive and negative bias and the corresponding values $I_{p +} = I(V_{p+}) > 0$ and $I_{p -} = I(V_{p-}) < 0$ of the current is all information needed as input. The arithmetic average of $V_{p +}$ and $\vert V_{p -}\vert$ in volt provides the value in electronvolt of the MO energy offset $\varepsilon_0 = E_{MO} - E_F$ relative to the electrode Fermi level ($\vert \varepsilon_0\vert = e (V_{p +} + \vert V_{p -}\vert )/2$). The value of the (Stark) strength of the bias-driven MO shift is obtained as $γ= (4/5) (V_{p +} - \vert V_{p -} \vert) / (V_{p +} + \vert V_{p -} \vert) $. Even the low-bias conductance estimate, $ G = (3/8) (I_{p +} / V_{p +} + I_{p -} / V_{p -})$, can be a preferable alternative to that deduced from fitting the $I$-$V$ slope in situations of noisy curves at low bias. To demonstrate the reliability and the generality of this ``five-thirds'' protocol, I illustrate its wide applicability for molecular tunnel junctions fabricated using metallic and nonmetallic electrodes, molecular species possessing localized $σ$ and delocalized $π$ electrons, and} various techniques (mechanically controlled break junctions, STM break junctions, conducting probe AFM junctions, and large area junctions).

cond-mat.mes-hall↗

arXiv2023

Can tunneling current in molecular junctions be so strongly temperature dependent to challenge a hopping mechanism? Analytical formulas answer this question and provide important insight into large area junctions

{Analytical equations like Richardson-Dushman's or Shockley's provided a general, if simplified conceptual background, which was widely accepted in conventional electronics and made a fundamental contribution to advances in the field. In the attempt to develop a (highly desirable, but so far missing) counterpart for molecular electronics, in this work, we deduce a general analytical formula for the tunneling current through molecular junctions mediated by a single level that is valid for any bias voltage and temperature. Starting from this expression, which is exact and obviates cumbersome numerical integration, in the low and high temperature limits we also provide analytical formulas expressing the current in terms of elementary functions. They are accurate for broad model parameter ranges relevant for real molecular junctions. Within this theoretical framework we show that: (i) by varying the temperature, the tunneling current can vary by several orders of magnitude, thus debunking the myth that a strong temperature dependence of the current is evidence for a hopping mechanism, (ii) real molecular junctions can undergo a gradual (Sommerfeld-Arrhenius) transition from a weakly temperature dependent to a strongly (``exponential'') temperature dependent current that can be tuned by the applied bias, and (iii) important insight into large area molecular junctions with eutectic gallium indium alloy (EGaIn) top electrodes can be gained. E.g., merely based on transport data, we estimate that the current carrying molecules represent only a fraction of f \approx 4 \times 10^{-4} out of the total number of molecules in a large area \ce{Au-S-(CH2)13-CH_3 / EGaIn} junction.

cond-mat.mes-hall↗

arXiv2023

Comment on "A single level tunneling model for molecular junctions: evaluating the simulation methods" by Opodi et al

The present Comment demonstrates important flaws of the paper Phys. Chem. Chem. Phys. 2022, 24, 11958 by Opodi~\emph{et al.} Their crown result (``applicability map'') aims at indicating parameter ranges wherein two approximate methods (called method 2 and 3) apply. My calculations reveal that the applicability map is a factual error. Deviations of $I_2$ from the exact current $I_1$ do not exceed 3\% for model parameters where Opodi~\emph{et al.} claimed that method 2 is inapplicable. As for method 3, the parameter range of the applicability map is beyond its scope, as stated in papers cited by Opodi~\emph{et al.}~themselves.

cond-mat.mes-hall↗

arXiv2023

Can room temperature data for tunneling molecular junctions be analyzed within a theoretical framework assuming zero temperature?

Routinely, experiments on tunneling molecular junctions report values of conductances ($G_{RT}$) and currents ($I_{RT}$) measured at room temperature. On the other side, theoretical approaches based on simplified models provide analytic formulas for the conductance ($G_{0K}$) and current ($I_{0K}$) valid at zero temperature. Therefore, interrogating the applicability of the theoretical results deduced in the zero temperature limit to real experimental situations at room temperature i.e., $G_{RT} \approx G_{0K}$ and $I_{RT} \approx I_{0K}$) is a relevant aspect. Quantifying the pertaining temperature impact on the transport properties computed within the ubiquitous single level model with Lorentzian transmission is the specific aim of the present work. Comprehensive results are presented for broad ranges of the relevant parameters (level's energy offset $\varepsilon_0$ and width $Γ_a $, and applied bias $V$) that safely cover values characterizing currently fabricated junctions. They demonstrate that the strongest thermal effects occur at biases below resonance ($2 \left\vert \varepsilon_0 \right\vert - δ\varepsilon_0 \alt \vert e V\vert \alt 2 \left\vert \varepsilon_0 \right\vert$). At fixed $V$, they affect an $\varepsilon_0$-range whose largest width $δ\varepsilon_0 $ is about nine times larger than the thermal energy ($δ\varepsilon_0 \approx 3 πk_B T$) at $Γ_a \to 0$. The numerous figures included aim at conveying a quick overview on the applicability of the zero temperature limit to a specific real junction. In quantitative terms, the conditions of applicability are expressed as mathematical inequalities involving elementary functions. They constitute the basis of an interactive data fitting procedure proposed, which aims at guiding experimentalists interested in data processing in a specific case.

cond-mat.mes-hall↗

arXiv2022

Estimating the Number of Molecules in Molecular Junctions Merely Based on the Low Bias Tunneling Conductance at Variable Temperature

Temperature ($T$) dependent conductance $G = G(T)$ data measured in molecular junctions are routinely taken as evidence for a two-step hopping mechanism. The present paper emphasizes that this is not necessarily the case. A curve of $\ln G$ versus $1/T$ decreasing almost linearly (Arrhenius-like regime) and eventually switching to a nearly horizontal plateau (Sommerfeld regime), or possessing a slope gradually decreasing with increasing $1/T$ is fully compatible with a single-step tunneling mechanism. The results for the dependence of $G$ on $T$ presented include both analytical exact and accurate approximate formulas and numerical simulations. These theoretical results are general, also in the sense that they are not limited, e.g., to the (single molecule electromigrated (SET) or large area EGaIn) fabrication platforms, which are chosen for exemplification merely in view of the available experimental data needed for analysis. To be specific, we examine in detail transport measurements for molecular junctions based on ferrocene (Fc). As a particularly important finding, we show how the present analytic formulas for $G=G(T)$ can be utilized to compute the ratio $f = A_{\text{eff}} / A_n$ between the effective and nominal areas of large area Fc-based junctions with an EGaIn top electrode. Our estimate of $f\approx 0.6 \times 10^{-4}$ is comparable with previously reported values \ib{based on completely different methods} for related large area molecular junctions.

cond-mat.mes-hall↗

arXiv2022

Exact analytic formula for conductance predicting a tunable Sommerfeld-Arrhenius thermal transition within a single-step tunneling mechanism in molecular junctions subject to mechanical stretching

We show that the conductance $G$ of molecular tunnel junctions wherein the charge transport is dominated by a single energy level can be expressed in closed analytic form which is exact and valid at arbitrary temperature $T$ and model parameter values. On this basis, we show that the single-step tunneling mechanism is compatible with a continuous thermal transition from a weakly $T$-dependent $G$ at low $T$ (Sommerfeld regime) to a nearly exponential $1/T$-dependent $G$ at high $T$ (Arrhenius-like regime). We predict that this Sommerfeld-Arrhenius transition can be observed in real molecular junctions % (e.g., based on perylene diimide) and can be continuously tuned, e.g., via mechanical stretching.

cond-mat.mes-hall↗

arXiv2022

Why Ortho- and Para-Hydroxy Metabolites Can Scavenge Free Radicals that the Parent Atorvastatin Cannot? Important Pharmacologic Insight from Quantum Chemistry

The pharmaceutical success of atorvastatin (ATV), a widely employed drug against the "bad" cholesterol (LDL) and cardiovascular diseases, traces back to its ability to scavenge free radicals. Unfortunately, information on its antioxidant properties is missing or unreliable. Here, we report detailed quantum chemical results for ATV and its ortho- and para-hydroxy metabolites (o-ATV, p-ATV) in the methanolic phase. They comprise global reactivity indices, bond order indices, and spin densities as well as all relevant enthalpies of reaction (bond dissociation BDE, ionization IP and electron attachment EA, proton detachment PDE and proton affinity PA, and electron transfer ETE). With these properties in hand, we can provide the first theoretical explanation of the experimental finding that, due to their free radical scavenging activity, ATV hydroxy metabolites rather than the parent ATV, have substantial inhibitory effect on LDL and the like. Surprisingly (because it is contrary to the most cases currently known), we unambiguously found that HAT (direct hydrogen atom transfer) rather than SPLET (sequential proton loss electron transfer) or SET-PT (stepwise electron transfer proton transfer) is the thermodynamically preferred pathway by which o-ATV and p-ATV in methanolic phase can scavenge DPPH$^\bullet$ (1,1-diphenyl-2-picrylhydrazyl) radicals. From a quantum chemical perspective, the ATV's species investigated are surprising because of the nontrivial correlations between bond dissociation energies, bond lengths, bond order indices and pertaining stretching frequencies, which do not fit the framework of naive chemical intuition.

physics.chem-ph↗

arXiv2022

Critical analysis of radical scavenging properties of atorvastatin in methanol recently estimated via density functional theory

In this communication we draw attention on serious flaws that plague recently reported antioxidant properties of atorvastatin (ATV) in methanol. First and foremost, we emphasize that the O-H bond dissociation energies (BDE) of about 400\,kcal/mol previously reported are completely wrong. Further, we present results refuting the previous claim that the proton affinity (PA) of ATV is smaller than that of the ascorbic acid. That unfounded claim relies on incorrect data for PA's ascorbic acid (which we correct here) circulated in the literature. Further, we correct the values of the chemical reactivity indices (e.g., chemical hardness, electrophilicity index, electroaccepting and electrodonating powers), which were inadequately estimated previously via Kohn-Sham HOMO and LUMO energies. Finally, our updated values for \ce{O-H} bond dissociation enthalpy (BDE = 91.4\,kcal/mol) and electron transfer enthalpy (ETE = 105.7\,kcal/mol) tentatively suggest that direct H-atom transfer (HAT) and sequential proton loss electron transfer (SPLET) may coexist.

physics.chem-ph↗

arXiv2021

Extensive Quantum Chemistry Study of Neutral and Charged C$_4$N Chains. An Attempt to Aid Astronomical Observation

Many molecular species can presumably still be observed in space if they are adequately characterized chemically. In this paper, we suggest that this could be the case of the neutral (C$_4$N$^0$) and anion (C$_4$N$^-$) cyanopropynylidene chains, which were not yet identified in space although both the neutral (C$_3$N$^0$ and C$_5$N$^0$) and anion (C$_3$N$^-$} and C$_5$N$^-$) neighboring members of the homologous series were observed. Extensive data obtained from quantum chemical calculations using density functional theory (DFT), coupled cluster (CC), and quadratic configuration interaction (QCI) methods for all charge and spin states of interest for space science (doublet and quartet neutrals, triplet and singlet anions, and singlet and triplet cations) are reported: e.g., bond metric and natural bond order data, enthalpies of formation, dissociation and reaction energies, spin gaps, rotational constants, vibrational properties, dipole and quadrupole momenta, electron attachment energies ($EA$) and ionization potentials ($IP$). The fact that (not only for C$_4$N but also for C$_2$N and C$_6$N) the quantum chemical methods utilized here are able to excellently reproduce the experimental $EA$ value -- which is often a challenge for theory -- is particularly encouraging, since this indicates that theoretical estimates of chemical reactivity indices (which are key input parameters for modeling astrochemical evolution) can be trusted. The presently calculated enthalpies of formation and dissociation energies do not substantiate any reason to assume that C$_4$N is absent in space. To further support this idea, we analyze potential chemical pathways of formation of both C$_4$N$^0$ and C$_4$N$^-$, which include association and exchange reactions.

physics.chem-ph↗

arXiv2021

Alternation of Singlet and Triplet States in Carbon-Based Chain Molecules and Its Astrochemical Implications. Results of an Extensive Theoretical Study

A variety of homologous carbon chains (HCnH, HCnN, CnS, CnO, and OCnO) are found to exhibit an appealing even-odd effect. Chains containing a number of carbon atoms of a certain parity possess singlet ground states, while members of opposite parity have triplet ground states. From a general perspective, it is important that this even-odd effect confounds straightforward chemical intuition. Whether the most stable form is a triplet or a singlet is neither simply related to the fact that the species in question is a normal (closed-shell, nonradical) molecule nor a (di)radical or to the (e.g., cumulene-type) C-C bond succession across the chain. From a computational perspective, the present results are important also because they demonstrate that electron correlations in carbon-based chains are extremely strong. Whether the gold-standard CCSD(T) (coupled-cluster expansions with single and double excitations and triple excitations corrections) framework suffices to describe such strongly correlated systems remains an open question that calls for further clarification. Most importantly for astrochemistry, the present results may explain why certain members are not astronomically observed although larger members of the same homologous series are detected; the missing species are exactly those for which the present calculations predict triplet ground states.

physics.chem-ph↗

arXiv2021

Why Asymmetric Molecular Coupling to Electrodes Cannot Be at Work in Real Molecular Rectifiers

Every now and then one can hear in the molecular electronics community that asymmetric couplings ($Γ_{s} \neq Γ_{t}$) of the dominant level (molecular orbital) to electrodes ($s$ and $t$) which typically have shapes different of each other may be responsible for current rectification observed in experiments. Using a general single level model going beyond the Lorentzian transmission limit, in this work we present a rigorous demonstration that this is not the case. In particular, we deduce an analytical for the bias ($V$) driven shift of the level energy $δ\varepsilon_{0}(V)$ showing that $δ\varepsilon_{0}(V)/V$ scales as $Γ_t/W_t - Γ_s/W_s$, which is merely a tiny quantity because the electrode bandwidths $W_{s,t}$ are much larger than $Γ_{s,t}$. This result invalidates a previous, never-deduced formula in use in some previous publications that neither could be justified theoretically nor is supported by experiment. To the latter aim, we present new experimental evidence adding to that already inferred in earlier analysis.

cond-mat.mes-hall↗

arXiv2020

Quantifying Inaccuracies in Modeling COVID-19 Pandemic within a Continuous Time Picture

Typically, mathematical simulation studies on COVID-19 pandemic forecasting are based on deterministic differential equations which assume that both the number ($n$) of individuals in various epidemiological classes and the time ($t$) on which they depend are quantities that vary continuous. This picture contrasts with the discrete representation of $n$ and $t$ underlying the real epidemiological data reported in terms daily numbers of infection cases, for which a description based on finite difference equations would be more adequate. Adopting a logistic growth framework, in this paper we present a quantitative analysis of the errors introduced by the continuous time description. This analysis reveals that, although the height of the epidemiological curve maximum is essentially unaffected, the position $T_{1/2}^{c}$ obtained within the continuous time representation is systematically shifted backwards in time with respect to the position $T_{1/2}^{d}$ predicted within the discrete time representation. Rather counterintuitively, the magnitude of this temporal shift $τ\equiv T_{1/2}^{c} - T_{1/2}^{d} < 0$ is basically insensitive to changes in infection rate $κ$. For a broad range of $κ$ values deduced from COVID-19 data at extreme situations (exponential growth in time and complete lockdown), we found a rather robust estimate $τ\simeq -2.65\,\mbox{day}^{-1}$. Being obtained without any particular assumption, the present mathematical results apply to logistic growth in general without any limitation to a specific real system.

physics.soc-ph↗

arXiv2020

Suppression of Groups Intermingling as Appealing Option For Flattening and Delaying the Epidemiological Curve While Allowing Economic and Social Life at Bearable Level During COVID-19 Pandemic

In this work, we simulate the COVID-19 pandemic dynamics in a population modeled as a network of groups wherein infection can propagate both via intra-group and via inter-group interactions. Our results emphasize the importance of diminishing the inter-group infections in the effort of substantial flattening and delaying of the epi(demiologic) curve with concomitant mitigation of disastrous economy and social consequences. To exemplify with a limiting case, splitting a population into m (say, 5 or 10) noninteracting groups while keeping intra-group interaction unchanged yields a stretched epidemiologic curve having the maximum number of daily infections reduced and postponed in time by the same factor $m$ (5 or 10). More generally, our study suggests a practical approach to fight against SARS-CoV-2 virus spread based on population splitting into groups and minimizing intermingling between them. This strategy can be pursued by large-scale infrastructure reorganization of activity at different levels in big logistic units (e.g., large productive networks, factories, enterprises, warehouses, schools, (seasonal) harvest work). Importantly, unlike total lockdwon strategy, the proposed approach prevents economic ruin and keeps social life at a more bearable level than distancing everyone from anyone. Last but not least, the declaration, for the first time in Europe, that COVID-19 epidemic ended in the two million people Slovenia may be taken as confirming the advantage of the strategy proposed in this paper.

physics.soc-ph↗

arXiv2020

What Can We Learn from the Time Evolution of COVID-19 Epidemic in Slovenia?

A recent work (DOI 10.1101/2020.05.06.20093310) indicated that temporarily splitting larger populations into smaller groups can efficiently mitigate the spread of SARS-CoV-2 virus. The fact that, soon afterwards, on May 15, 2020, the two million people Slovenia was the first European country proclaiming the end of COVID-19 epidemic within national borders may be relevant from this perspective. Motivated by this evolution, in this paper we investigate the time dynamics of coronavirus cases in Slovenia with emphasis on how efficient various containment measures act to diminish the number of COVID-19 infections. Noteworthily, the present analysis does not rely on any speculative theoretical assumption; it is solely based on raw epidemiological data. Out of the results presented here, the most important one is perhaps the finding that, while imposing drastic curfews and travel restrictions reduce the infection rate kappa by a factor of four with respect to the unrestricted state, they only improve the \k{appa}-value by ~15 % as compared to the much bearable state of social and economical life wherein (justifiable) wearing face masks and social distancing rules are enforced/followed. Significantly for behavioral and social science, our analysis of the time dependence \k{appa} = \k{appa}(t) may reveal an interesting self-protection instinct of the population, which became manifest even before the official lockdown enforcement.

physics.soc-ph↗

arXiv2015

Counterintuitive issues in the charge transport through molecular junctions

Whether at phenomenological or microscopic levels, most theoretical approaches to charge transport through molecular junctions postulate or attempt to justify microscopically the existence of a dominant molecular orbital (MO). Within such single level descriptions, experimental current-voltage I-V curves are sometimes/often analyzed by using analytical formulas expressing the current as a cubic expansion in terms of the applied voltage V, and relate possible V-driven shifts of the level energy offset relative to the metallic Fermi energy \varepsilon_{0} to an asymmetry of molecule-electrode couplings or to an asymmetric location of the "center of gravity" of the MO with respect to electrodes. In this paper, we present results demonstrating the failure of these intuitive expectations. For example, we show how typical data processing based on cubic expansions yields a value of \varepsilon_0 underestimated by a typical factor of about two. When compared to theoretical results of DFT approaches, which typically underestimate the HOMO-LUMO gap by a similar factor, this may create the false impression of "agreement" with experiments in situations where this is actually not the case. Further, such cubic expansions yield model parameter values dependent on the bias range width employed for fitting, which is unacceptable physically. Finally, we present an example demonstrating that, counter-intuitively, the bias-induced change in the energy of an MO located much closer to an electrode can occur in a direction that is opposite to the change in the Fermi energy of that electrode. This is contrary to what one expects based on a "lever rule" argument, according to which the MO "feels" the local value of the electric potential, which is assumed to vary linearly across the junction and is closer to the potential of the closer electrode.

cond-mat.mes-hall↗

arXiv2015

Quantifying the relative molecular orbital alignment for molecular junctions with similar chemical linkage to electrodes

Estimating the relative alignment between the frontier molecular orbitals that dominates the charge transport through single-molecule junctions represents a challenge for theory. This requires approaches beyond the widely employed framework provided by the density functional theory, wherein the Kohn-Sham "orbitals" are treated as if they were real molecular orbitals, which is not the case. In this paper, we report results obtained by means of quantum chemical calculations, including the EOM-CCSD (equation-of-motion coupled-cluster singles and doubles), which is the state-of-the-art of quantum chemistry for medium-size molecules like those considered here. These theoretical results are validated against data on the molecular orbital energy offset relative to the electrodes' Fermi energy extracted from experiments for junctions based on 4,4'-bipyridine and 1,4-dicyanobenzene.

cond-mat.mes-hall↗

arXiv2015

Electrochemical setup - a unique chance to simultaneously control orbital energies and vibrational properties of single-molecule junctions with unprecedented efficiency

Impressive advances in nanoscience permit nowadays to manipulate single molecules and broadly control many of their properties. Still, tuning the molecular charge and vibrational properties of single molecules embedded in nanojunctions in broad ranges escaped so far to an efficient control. By combining theoretical results with recent experimental data, we show that, under electrochemical control, it is possible to continuously drive a redox molecule (viologen) between almost perfect oxidized and reduced states. This yields an unprecedentedly efficient control on both vibrational frequencies and the surface-enhanced Raman scattering (SERS) intensities. The broad tuning achieved under electrochemical control by varying the overpotential ("gate potential") within experimentally accessible ranges contrasts to the case of two-terminal setups that require high biases, which real nanojunctions cannot withstand. The present study aim at stimulating concurrent transport and SERS measurements in electrochemical setup. This may open a new avenue of research that is not accessible via two-terminal approaches for better understanding the transport at nanoscale.

physics.chem-ph↗