skip to main content

Title: Ultrasensitive Photothermal Spectroscopy: Harnessing the Seebeck Effect for Attogram-Level Detection
Molecular-level spectroscopy is crucial for sensing and imaging applications, yet detecting and quantifying minuscule quantities of chemicals remains a challenge, especially when they surface-adsorb in low numbers. Here, we introduce a photothermal spectroscopic technique that enables the sensing and quantification of adsorbates with an attogram detection limit. Our approach utilizes the Seebeck effect in a microfabricated nanoscale thermocouple junction, incorporated into the apex of a microcantilever. We observe minimal thermal mass exhibited by the sensor which maintains exceptional thermal insulation. The temperature variation driving the thermoelectric junction arises from the non-radiative decay of molecular adsorbates' vibrational states on the tip. We demonstrate the detection of physisorbed trinitrotoluene (TNT) and dimethyl methylphosphonate (DMMP) molecules, as well as representative polymers, with an estimated mass sensitivity of 10-18 g and a temperature resolution of 40 mK.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ; ;
Date Published:
Journal Name:
Nano Letters
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. null (Ed.)
    Optically excited plasmonic nanostructures exhibit unique capabilities to catalyze interfacial chemical transformations of molecules adsorbed on their surfaces in a regioselective manner through anomalous reaction pathways that are inaccessible under thermal conditions. The mechanistic complexity of plasmon-driven photocatalysis is intimately tied to a series of photophysical and photochemical processes associated with the radiative and non-radiative decay of localized plasmon resonances in metallic nanostructures. Plasmon-enhanced Raman spectroscopy combines ultrahigh detection sensitivity with unique time-resolving and molecular finger-printing capabilities, ideal for detailed kinetic and mechanistic studies of photocatalytic interfacial transformations of molecular adsorbates residing in the plasmonic hot spots. Through systematic case studies of several representative reactions, we demonstrate how plasmon-enhanced Raman spectroscopy can be judiciously utilized as a unique in situ spectroscopic tool to fine-resolve the detailed molecule-transforming processes on the surfaces of optically excited plasmonic nanostructures in real time during the photocatalytic reactions. We further epitomize the mechanistic insights gained from in situ plasmon-enhanced Raman spectroscopic measurements into several central materials design principles that can be employed to guide the rational optimization of the photocatalyst structures and the nanostructure-molecule interfaces for plasmon-mediated surface chemistry. 
    more » « less
  2. With the objective of understanding microscopic principles governing thermal energy flow in nanojunctions, we study phononic heat transport through metal-molecule-metal junctions using classical molecular dynamics (MD) simulations. Considering a single-molecule gold-alkanedithiol-gold junction, we first focus on aspects of method development and compare two techniques for calculating thermal conductance: (i) The Reverse Nonequilibrium MD (RNEMD) method, where heat is inputted and extracted at a constant rate from opposite metals. In this case, the thermal conductance is calculated from the nonequilibrium temperature profile that is created at the junction. (ii) The Approach-to-Equilibrium MD (AEMD) method, with the thermal conductance of the junction obtained from the equilibration dynamics of the metals. In both methods, simulations of alkane chains of a growing size display an approximate length-independence of the thermal conductance, with calculated values matching computational and experimental studies. The RNEMD and AEMD methods offer different insights, and we discuss their benefits and shortcomings. Assessing the potential application of molecular junctions as thermal diodes, alkane junctions are made spatially asymmetric by modifying their contact regions with the bulk, either by using distinct endgroups or by replacing one of the Au contacts with Ag. Anharmonicity is built into the system within the molecular force-field. We find that, while the temperature profile strongly varies (compared with the gold-alkanedithiol-gold junctions) due to these structural modifications, the thermal diode effect is inconsequential in these systems—unless one goes to very large thermal biases. This finding suggests that one should seek molecules with considerable internal anharmonic effects for developing nonlinear thermal devices. 
    more » « less
  3. Abstract

    Whether the presence of adsorbates increases or decreases thermal conductivity in metal-organic frameworks (MOFs) has been an open question. Here we report observations of thermal transport in the metal-organic framework HKUST-1 in the presence of various liquid adsorbates: water, methanol, and ethanol. Experimental thermoreflectance measurements were performed on single crystals and thin films, and theoretical predictions were made using molecular dynamics simulations. We find that the thermal conductivity of HKUST-1 decreases by 40 – 80% depending on the adsorbate, a result that cannot be explained by effective medium approximations. Our findings demonstrate that adsorbates introduce additional phonon scattering in HKUST-1, which particularly shortens the lifetimes of low-frequency phonon modes. As a result, the system thermal conductivity is lowered to a greater extent than the increase expected by the creation of additional heat transfer channels. Finally, we show that thermal diffusivity is even more greatly reduced than thermal conductivity by adsorption.

    more » « less
  4. Chiral metamaterials in the mid-infrared wavelength range have tremendous potential for studying thermal emission manipulation and molecular vibration sensing. Here, we present one type of chiral plasmonic metasurface absorber with high circular dichroism (CD) in absorption of more than 0.56 across the mid-infrared wavelength range of 5–5.5 µm. The demonstrated chiral metasurface absorbers exhibit a maximum chiral absorption of 0.87 and a maximum CD in absorption of around 0.60. By adjusting the geometric parameters of the unit cell structure of the metasurface, the chiral absorption peak can be shifted to different wavelengths. Due to the strong chiroptical response, the thermal analysis of the designed chiral metasurface absorber further shows the large temperature difference between the left-handed and right-handed circularly polarized light. The demonstrated results can be utilized in various applications such as molecular detection, mid-infrared filter, thermal emission, and chiral imaging.

    more » « less
  5. Paramagnetic single-molecule magnets (SMMs) interacting with the ferromagnetic electrodes of a magnetic tunnel junction (MTJ) produce a new system. The properties and future scope of new systems differ dramatically from the properties of isolated molecules and ferromagnets. However, it is unknown how far deep in the ferromagnetic electrode the impact of the paramagnetic molecule and ferromagnet interactions can travel for various levels of molecular spin states. Our prior experimental studies showed two types of paramagnetic SMMs, the hexanuclear Mn 6 and octanuclear Fe–Ni molecular complexes, covalently bonded to ferromagnets produced unprecedented strong antiferromagnetic coupling between two ferromagnets at room temperature leading to a number of intriguing observations (P. Tyagi, et al. , Org. Electron. , 2019, 64 , 188–194. P. Tyagi, et al. , RSC Adv. , 2020, 10 , (22), 13006–13015). This paper reports a Monte Carlo Simulations (MCS) study focusing on the impact of the molecular spin state on a cross junction shaped MTJ based molecular spintronics device (MTJMSD). Our MCS study focused on the Heisenberg model of MTJMSD and investigated the impact of various molecular coupling strengths, thermal energy, and molecular spin states. To gauge the impact of the molecular spin state on the region of ferromagnetic electrodes, we examined the spatial distribution of molecule-ferromagnet correlated phases. Our MCS study shows that under a strong coupling regime, the molecular spin state should be ∼30% of the ferromagnetic electrode's atomic spins to create long-range correlated phases. 
    more » « less