arXiv subjects

Jouni Ahopelto

Publications and source records attributed to Jouni Ahopelto.

12 recordsLinked to original sources

arXiv2026

Dynamics of confined acoustic phonon modes in nanocrystalline silicon membranes

We report femtosecond time-resolved measurements of confined acoustic phonons in free-standing nanocrystalline silicon membranes and compare them with the crystalline silicon counterpart. While the latter exhibit well-resolved higher-order modes, a strong suppression of these modes is observed in nanocrystalline samples. The suppression is strongly frequency dependent and becomes more pronounced as the phonon wavelength approaches the characteristic grain size. Analysis of the phonon lifetimes reveals an additional frequency-dependent scattering mechanism in nanocrystalline silicon, with a scattering rate that scales approximately as $f^2$, where $f$ is the phonon frequency. The measured sound velocity is consistent with previous reports for nanocrystalline silicon and indicates an effective elastic response arising from multiple crystallographic orientations. These results establish coherent phonons as a sensitive probe of microstructure-dependent scattering in nanocrystalline materials and indicate that grain boundaries act as an effective spectral filter for high-frequency acoustic phonons.

physics.optics↗

arXiv2021

Thermal properties of nanocrystalline silicon nanobeams

Controlling thermal energy transfer at the nanoscale has become critically important in many applications and thermal properties since it often limits device performance. In this work, we study the effects on thermal conductivity arising from the nanoscale structure of free-standing nanocrystalline silicon films and the increasing surface-to-volume ratio when fabricated into suspended optomechanical nanobeams. We characterize thermal transport in structures with different grain sizes and elucidate the relative impact of grain size and geometrical dimensions on thermal conductivity. We use a micro-time-domain thermoreflectance method to study the impact of the grain size distribution, from 10 to 400 nm, on the thermal conductivity in free-standing nanocrystalline silicon films considering surface phonon and grain boundary scattering. We find a drastic reduction in the thermal conductivity, down to values of 10 W.m^{-1}.K^{-1} and below, which is just a fraction of the conductivity of single crystalline silicon. Decreasing the grain size further decreases the thermal conductivity. We also observe that this effect is smaller in OM nanostructures than in membranes due to the competition of surface scattering in decreasing thermal conductivity. Finally, we introduce a novel versatile contactless characterization technique that can be adapted to any structure supporting a thermally shifted optical resonance and use it to evaluate the thermal conductivity. This method can be used with optical resonances exhibiting different mode profiles and the data is shown to agrees quantitatively with the thermoreflectance measurements. This work opens the way to a more generalized thermal characterization of optomechanical cavities and to create hot-spots with engineered shapes at desired position in the structures as a means to study thermal transport in coupled photon-phonon structures.

physics.app-ph↗

arXiv2021

Nano-thermoelectric infrared bolometers

Infrared (IR) radiation detectors are used in numerous applications from thermal imaging to spectroscopic gas sensing. Obtaining high speed and sensitivity, low-power operation and cost-effectiveness with a single technology remains to be a challenge in the field of IR sensors. By combining nano-thermoelectric transduction and nanomembrane photonic absorbers, we demonstrate uncooled IR bolometer technology that is material-compatible with large-scale CMOS fabrication and provides fast and high sensitivity response to long-wavelength IR (LWIR) around 10 $μ$m. The fast operation speed stems from the low heat capacity metal layer grid absorber connecting the sub-100 nm-thick n- and p-type Si nano-thermoelectric support beams, which convert the radiation induced temperature rise into voltage. The nano-thermoelectric transducer-support approach benefits from enhanced phonon surface scattering in the beams leading to reduction in thermal conductivity, which enhances the sensitivity. We demonstrate different size nano-thermoelectric bolometric photodetector pixels with LWIR responsitivities, specific detectivities and time constants in the ranges 179-2930 V/W, 0.15-3.1$\cdot10^{8}$ cmHz$^{1/2}$/W and 66-3600 $μ$s, respectively. We benchmark the technology against different LWIR detector solutions and show how nano-thermoelectric detector technology can reach the fundamental sensitivity limits posed by phonon and photon thermal fluctuation noise.

physics.app-ph↗

arXiv2020

Properties of Nanocrystalline Silicon Probed by Optomechanics

Nanocrystalline materials exhibit properties that can differ substantially from those of their single crystal counterparts. As such, they provide ways to enhance and optimise their functionality for devices and applications. Here we report on the optical, mechanical and thermal properties of nanocrystalline silicon probed by means of optomechanical nanobeams to extract information of the dynamics of optical absorption, mechanical losses, heat generation and dissipation. The optomechanical nanobeams are fabricated using nanocrystalline films prepared by annealing amorphous silicon layers at different temperatures. The resulting crystallite sizes and the stress in the films can be controlled by the annealing temperature and time and, consequently, the properties of the films can be tuned relatively freely, as demonstrated here by means of electron microscopy and Raman scattering. We show that the nanocrystallite size and the volume fraction of the grain boundaries play a key role in the dissipation rates through non-linear optical and thermal processes. Promising optical (13000) and mechanical (1700) quality factors were found in the optomechanical cavity realised in the nanocrystalline Si resulting from annealing at 950 C. The enhanced absorption and recombination rates via the intra-gap states and the reduced thermal conductivity boost the potential to exploit these non-linear effects in applications, including NEMS, phonon lasing and chaos-based devices.

physics.app-ph↗

arXiv2020

Electronic Band Structures of Pristine and Chemically Modified Cellulose Allomorphs

We have investigated the structural properties, vibrational spectra, and electronic band structures of crystalline cellulose allomorphs and chemically modified cellulose with quantum chemical methods. The electronic band gaps of cellulose allomorphs I$_α$, I$_β$, II, and III$_1$ lie in the range of 5.0 to 5.6 eV. We show that extra states can be created in the band gap of cellulose by chemical modification. Experimentally feasible amidation of cellulose I$_β$ with aniline or 4,4'- diaminoazobenzene creates narrow bands in the cellulose band gap, reducing the difference between the occupied and empty states to 4.0 or 1.8 eV, respectively. The predicted states 4,4'-diaminoazobenzene-modified cellulose I$_β$ fall in the visible spectrum, suggesting uses in optical applications.

cond-mat.mtrl-sci↗

arXiv2017

Thermoelectric bolometers based on ultra-thin heavily doped single-crystal silicon membranes

We present ultra-thin silicon membrane thermocouple bolometers suitable for fast and sensitive detection of low levels of thermal power and infrared radiation at room temperature. The devices are based on 40 nm-thick strain tuned single crystalline silicon membranes shaped into heater/absorber area and narrow n- and p-doped beams, which operate as the thermocouple. The electro-thermal characterization of the devices reveal noise equivalent power of 13 pW/rtHz and thermal time constant of 2.5 ms. The high sensitivity of the devices is due to the high Seebeck coefficient of 0.39 mV/K and reduction of thermal conductivity of the Si beams from the bulk value. The bolometers operate in the Johnson-Nyquist noise limit of the thermocouple, and the performance improvement towards the operation close to the temperature fluctuation limit is discussed.

cond-mat.mes-hall↗

arXiv2016

Conformal Titanium Nitride in a Porous Silicon Matrix: a Nanomaterial for In-Chip Supercapacitors

Today's supercapacitor energy storages are typically discrete devices aimed for printed boards and power applications. The development of autonomous sensor networks and wearable electronics and the miniaturisation of mobile devices would benefit substantially from solutions in which the energy storage is integrated with the active device. Nanostructures based on porous silicon (PS) provide a route towards integration due to the very high inherent surface area to volume ratio and compatibility with microelectronics fabrication processes. Unfortunately, pristine PS has limited wettability and poor chemical stability in electrolytes and the high resistance of the PS matrix severely limits the power efficiency. In this work, we demonstrate that excellent wettability and electro-chemical properties in aqueous and organic electrolytes can be obtained by coating the PS matrix with an ultra-thin layer of titanium nitride by atomic layer deposition. Our approach leads to very high specific capacitance (15 F/cm$^3$), energy density (1.3 mWh/cm$^3$), power density (up to 214 W/cm$^3$) and excellent stability (more than 13,000 cycles). Furthermore, we show that the PS-TiN nanomaterial can be integrated inside a silicon chip monolithically by combining MEMS and nanofabrication techniques. This leads to realisation of in-chip supercapacitor, i.e., it opens a new way to exploit the otherwise inactive volume of a silicon chip to store energy.

cond-mat.mtrl-sci↗

arXiv2015

Optical and mechanical properties of nanofibrillated cellulose: towards a robust platform for next-generation green technologies

Nanofibrillated cellulose, a polymer that can be obtained from one of the most abundant biopolymers in Nature, is being increasingly explored due to its outstanding properties for packaging and device applications. Still, open challenges in engineering its intrinsic properties remain to address. The results obtained show the precise determination of significant properties as elastic properties and interactions that are compared with similar works and, moreover, demonstrate that nanofibrillated cellulose properties can be reversibly controlled, supporting the extended potential of nanofibrillated cellulose as a robust platform for green-technology applications

cond-mat.mtrl-sci↗

arXiv2014

Reconstructing phonon mean free path contributions to thermal conductivity using nanoscale membranes

Knowledge of the mean free path distribution of heat-carrying phonons is key to understanding phonon-mediated thermal transport. We demonstrate that thermal conductivity measurements of thin membranes spanning a wide thickness range can be used to characterize how bulk thermal conductivity is distributed over phonon mean free paths. A non-contact transient thermal grating technique was used to measure the thermal conductivity of suspended Si membranes ranging from 15 to 1500 nm in thickness. A decrease in the thermal conductivity from 74% to 13% of the bulk value is observed over this thickness range, which is attributed to diffuse phonon boundary scattering. Due to the well-defined relation between the membrane thickness and phonon mean free path suppression, combined with the range and accuracy of the measurements, we can reconstruct the bulk thermal conductivity accumulation vs. phonon mean free path, and compare with theoretical models.

cond-mat.mes-hall↗

arXiv2013

Ultra-Thin Free-Standing Single Crystalline Silicon Membranes With Strain Control

We report on fabrication and characterization of ultra-thin suspended single crystalline flat silicon membranes with thickness down to 6 nm. We have developed a method to control the strain in the membranes by adding a strain compensating frame on the silicon membrane perimeter to avoid buckling of the released membranes. We show that by changing the properties of the frame the strain of the membrane can be tuned in controlled manner. Consequently, both the mechanical properties and the band structure can be engineered and the resulting membranes provide a unique laboratory to study low-dimensional electronic, photonic and phononic phenomena.

cond-mat.mes-hall↗

arXiv2012

Phonons in Slow Motion: Dispersion Relations in Ultra-Thin Si Membranes

We report the changes in dispersion relations of hypersonic acoustic phonons in free-standing silicon membranes as thin as \sim 8 nm. We observe a reduction of the phase and group velocities of the fundamental flexural mode by more than one order of magnitude compared to bulk values. The modification of the dispersion relation in nanostructures has important consequences for noise control in nano and micro-electromechanical systems (MEMS/NEMS) as well as opto-mechanical devices.

cond-mat.mes-hall↗

arXiv2005

Two sub-band conductivity of Si quantum well

We report on two sub-band transport in double gate SiO$_2$-Si-SiO$_2$ quantum well with 14 nm thick Si layer at 270 mK. At symmetric well potential the experimental sub-band spacing changes monotonically from 2.3 to 0.3 meV when the total density is adjusted by gate voltages between $\sim 0.7\times 10^{16}$ $-3.0\times 10^{16}$ m$^{-2}$. The conductivity is mapped in large gate bias window and it shows strong non-monotonic features. At symmetric well potential and high density these features are addressed to sub-band wave function delocalization in the quantization direction and to different disorder of the top and bottom interfaces of the Si well. Close to bi-layer/second sub-band threshold the non-monotonic behavior is interpreted to arise from scattering from localized band tail electrons.

cond-mat.dis-nn↗